First note that just because something can be explained by YT tracking you in incognito mode, this does not mean that this actually happening. Just because there is an apparent explanation does not mean that this is the only explanation or that it is the correct one. Note also that you have a biased perspective, i.e. you argue based on a single video which you've watched in incognito mode and which got recommended to you, but ignore all the videos you watched in incognito mode and which did not get recommended to you.
I doubt that YT actually knows what you've already watched in incognito mode and therefore recommends you the same thing again. But YT (or Google in general) tracks your general interests, not only on YT itself but over many sites using Google Analytics embedded in a significant part of the web pages.
It is probably correct to assume, that watching this video somehow aligns with your interests. And that you've implicitly shown these interests by watching other videos on YT or by visiting other sites while not being in incognito mode. Or the video was included in a site you've visited recently. So the recommendation does not completely come out of the blue and there is no need to actually track you in incognito mode to recommend you this video.
Answer from Steffen Ullrich on Stack ExchangeVideos
First note that just because something can be explained by YT tracking you in incognito mode, this does not mean that this actually happening. Just because there is an apparent explanation does not mean that this is the only explanation or that it is the correct one. Note also that you have a biased perspective, i.e. you argue based on a single video which you've watched in incognito mode and which got recommended to you, but ignore all the videos you watched in incognito mode and which did not get recommended to you.
I doubt that YT actually knows what you've already watched in incognito mode and therefore recommends you the same thing again. But YT (or Google in general) tracks your general interests, not only on YT itself but over many sites using Google Analytics embedded in a significant part of the web pages.
It is probably correct to assume, that watching this video somehow aligns with your interests. And that you've implicitly shown these interests by watching other videos on YT or by visiting other sites while not being in incognito mode. Or the video was included in a site you've visited recently. So the recommendation does not completely come out of the blue and there is no need to actually track you in incognito mode to recommend you this video.
Recommendations based on IP address can still work. They don't have to confirm whether that incognito user is really you. They can assume that even if it's not you, it is atleast someone who is closer to you. So they can recommend that to anyone behind that IP address including you.
Recommendations also depends on your watch history and how likely you will click to see it. Together with browser fingerprinting, it can produce more accurate results. Incognito doesn't anonymize you from web service. It only separates your private session from persistent ones.
I mean we all know what these things are used for and YouTube has none of that. So what do anyone need Incagnito mode for on YouTube???
I've actually started to notice this for about 6 months maybe more. But I didn't bother doing some research on this.
So, what happens is that sometimes if I don't want my YouTube recommendations page to be cluttered with topics I am much less interested, I would watch that video in incognito. Guess what? After some time, similar videos start to pop up on my recommendations page. And I know I 100% not watched it on normal mode. And this is so subtle, that it doesn't happen overnight. It takes days, maybe even weeks. So, when it happens you wouldn't probably remember in 100% certainty that you watched it in incognito. This may sound a bit crazy, but this happened to me quite a few times that I decided to do some research and post it here. And one more thing. If I watch something in other language (Russian), such videos then don't populate YouTube's recommendation page. Of course, if it did, that would be too obvious.
I did some search on duckduckgo.com and https://tech4more.com/google-tracking-incognito-mode/ found this article. Well, it doesn't talk about YouTube recommendation thing, but was a quite interesting read for me. And one point it mentions is that "If you truly want to stay Incognito while using the Incognito Mode, you have to make sure that you have logged out of your Google account before activating it."
Now, one thing baffles me. I am using chromium. And it's free and open-source. Now, how does Google manages to track with open-source software? Some security enthusiast could eventually study the code and find this out. Am I missing something?