CapitalOne is using the day after the date you have listed on your spreadsheet.

For example, for your first transaction of 43 SGD, you have a date of 12-Oct-2018. However, if you plug 13-Oct-2018 into Visa's Exchange Rate Calculator, you'll see that it gives you the same USD price as CapitalOne charged you.

The discrepancy in date might be related to the time difference, as you noted.

In my opinion, it is nothing to worry about. As you noted, it is just as likely as not to work in your favor. For cards that do have a foreign exchange fee, the fee averages 3%, and the difference you are seeing is nowhere close to that.

In response to your edit:

Please also explain why the three transactions on Nov 2nd have slightly different exchange rates even on my credit card statement.

All three of these transactions are using the exchange rate from November 3.

It appears that CapitalOne is using the real exchange rate for November 3 for each of these transactions. They then round to the nearest penny. Finally, for the exchange rate that appears on your statement, they calculate it for each transaction using the rounded amount that you were actually charged.

Example: The 11/2/2018 transaction of 30.00 SGD is converted to $21.902010 USD based on the 11/3/2018 rate. This is then rounded to $21.90, and is what you were charged on your credit card account. For the exchange rate listed on your statement, they simply calculated 30.00/21.90 = 1.369863014. Because of the rounding to the nearest penny, the exchange rates calculated on your statement in the last step are all slightly different from each other, but notice that all three of them are identical to the first three decimal places. Yes, it might be less confusing if they simply showed you the actual rate used in the first step, rather then back-calculating the rate after the rounding occurs, but that is the choice they made, and most people won't care.