Update: I figured out part of the problem when I realized that the events for the two google calendars were the same color. (I don't know if that is by default. If so, it should be changed.) When I assigned distinct colors for each calendar, it became obvious that some events were associated with the "Home" calendar. I had not noticed this calendar before and had not knowingly used it. Now I just need to figure out how to prevent it from being used again (and have posted a separate question about this). I hope that this information is helpful to someone else.