Quality of recorded zoom call

I`d like to know why the quality of recorded zoom call changes even within one call. Sometimes the image from my web cam is quite good, and sometimes it is not. In the attached files there are two screenshots that were taken from one recorded call, all the settings were the same. What is the issue? Is it my internet speed or something else? I use Zoom on PC, version 5.15.7

 Any help and advice on how to improve the quality of video would be appreciated