You can't just use total time played divided by games played and get a very meaningful number.
You don't know how much time has been spent in particular difficulties.
You don't know the typical group makeup of each game (frequent PUGging or soloing will increase average time, always playing with friends probably decreases it).
You don't know how many of those games were spent just stuffing around for one wave to quickly test out something or other.
My total time is 43491 minutes, divided by 2458 games, for an average of 17.69 minutes per game.
But that is much shorter than my average time for an UUG PUG extraction (pretty much all that I play), which is roughly 23 minutes.
There's arguably three "KINDS" of averages, but the typical is the MEAN average, which is EXACTLY "Add it all up, divide by the number, and boom." This would encompass all our games, ever, and give us the results we're getting. Wild outliers have a fairly minimal effect on the Mean Average once the sample size is high enough (Considering most of us have thousands of games under our belt, the odd 5 minute bleed out or 50 minute solo doesn't have THAT much of an effect) so it really is a very reliable number.
You're thinking more of the MODE average, which is just "whatever reccurs the most." Got 100 numbers, and 23 appears 30 times while the second highest is 11 at 20 times? The "Mode" average is 23 and every other number that appears can get stuffed. The mode is generally more useful in things like business, like "Okay, we generally rake in this much on Fridays, this much on Saturdays, etc" where a more recent trend of stats would be more useful in the short term, and completely ignores outliers.
Last, and I have no idea where this would even be useful, is the MEDIAN. Let's say you have 18802 different numbers between 0 and 100, but at least one of those numbers is 0 and one of those is 100? The median is 50. It's the half way point between the lowest and highest number. Got some wild outliers? They dictate everything. If you have a mean average range of 20, for example, while you have outliers of as low as 5 and as high as one singular 3000, your median will be over 1000, which is insane!
TLDR: We're using the mean average, generally accepted as the mathematical "true" average. Math does not lie. Unless you do it wrong.