Short answer
This is because params sends the parameters as part of the http POST request, while data sends them as part of the body of the request. In your case: just call the api using params and you're fine. This is absolutely normal (and expected) behaviour.

Demonstration
Just start two commandlines. On the first, run netcat: nc -l 8888. On the other commandline, run python:

>>> import requests
>>> requests.post('http://localhost:8888',data={'a':1,'b':'2'})

At the netcat-side, we see the following request:

POST / HTTP/1.1
Host: localhost:8888
Connection: keep-alive
Accept-Encoding: gzip, deflate
Accept: */*
User-Agent: python-requests/2.18.1
Content-Length: 7
Content-Type: application/x-www-form-urlencoded

a=1&b=2

Next, try the params way:

>>> requests.post('http://localhost:8888',params={'a':1,'b':'2'})

Netcat reports:

POST /?a=1&b=2 HTTP/1.1
Host: localhost:8888
Connection: keep-alive
Accept-Encoding: gzip, deflate
Accept: */*
User-Agent: python-requests/2.18.1
Content-Length: 0

Note the differences in the first and last line.

As you can read from the documentation (italic emphasis is mine):

params -- (optional) Dictionary or bytes to be sent in the query string for the Request.
data -- (optional) Dictionary or list of tuples [(key, value)] (will be form-encoded), bytes, or file-like object to send in the body of the Request.

Short answer
This is because params sends the parameters as part of the http POST request, while data sends them as part of the body of the request. In your case: just call the api using params and you're fine. This is absolutely normal (and expected) behaviour.

Demonstration
Just start two commandlines. On the first, run netcat: nc -l 8888. On the other commandline, run python:

>>> import requests
>>> requests.post('http://localhost:8888',data={'a':1,'b':'2'})

At the netcat-side, we see the following request:

POST / HTTP/1.1
Host: localhost:8888
Connection: keep-alive
Accept-Encoding: gzip, deflate
Accept: */*
User-Agent: python-requests/2.18.1
Content-Length: 7
Content-Type: application/x-www-form-urlencoded

a=1&b=2

Next, try the params way:

>>> requests.post('http://localhost:8888',params={'a':1,'b':'2'})

Netcat reports:

POST /?a=1&b=2 HTTP/1.1
Host: localhost:8888
Connection: keep-alive
Accept-Encoding: gzip, deflate
Accept: */*
User-Agent: python-requests/2.18.1
Content-Length: 0

Note the differences in the first and last line.

As you can read from the documentation (italic emphasis is mine):

params -- (optional) Dictionary or bytes to be sent in the query string for the Request.
data -- (optional) Dictionary or list of tuples [(key, value)] (will be form-encoded), bytes, or file-like object to send in the body of the Request.