Why did my Basic meeting terminate with 7 minutes still remaining on my countdown?

I have a Basic account, and host a scheduled weekly meeting. Anyone with the link can join. No waiting room. This week, the meeting terminated unexpectedly early - there was about 7 minutes remaining on MY countdown timer. I got the big 'Need more time?' popup message at the usual time - around 30 minutes in. I suspect that it was because someone entered the meeting 10 minutes early, then left the meeting early (that was the only difference I know of compared to other meetings that lasted 40 minutes).  Oh, and this week I dismissed the popup message (I usually just leave it there). There were no noticeable connection issues. Can someone shed any light on this, specifically what should I do to ensure I get the full 40 minutes? Should I perhaps use the waiting room to prevent people from joining early and starting the timer? Thank you in advance for any helpful hints!