That's certainly true. And is also why I was just averaging things rather than try and be overly precise. 3-5 rounds and 6-8 encounters came out to 7 encounters of 4 rounds each.
What one means by medium, hard and deadly is in the DMG, though. It's the XP budget of the fight, although deadly doesn't work out as well since if hit deadly with 2 minotaurs you will still be at deadly with 200 minotaurs, but clearly one is much deadlier than the other. Fight durations will vary much more due to the range of what is deadly.