If I run it once, it may return 40 results. If I go to the "next page" of results (increment the start parameter by 10), it may say 49 results... or 21 results... It's all over the place.

I believe that google indexes might have been updated during that time and start (offset) has been reset.

google don't have definite fixed time when they update indexes, they constantly update their indexes https://support.google.com/websearch/answer/12412910?hl=en