From what I can tell, the game tends to bombard you with your most recently "unlocked" call. I.e. you get lots of Hazmat right after you build your 11th fire station and a lot of SWAT after your 8th police station.
On the topic of realism, however, I agree with the "this is a game" perspective. In my area, entire days will go by without a single call. Period. While I take a realistic approach to apparatus and stations, I greatly appreciate the high call volume as it keeps me entertained. Not to say that I don't want there to be some believability (I get industrial fires in places where it's just a wheatfield XD), but I also enjoy thinking about how my area would respond to such an unbelievable series of calls.
This particular call has raised some concern for me lately... To put it simply - it occurs/generates far too often to be considered even close to being realistic.
The calls are randomized, there is every chance you can get more of one type of call than the other for any period, this is not a game bug, just how a computer understands randomization.
If the game goes on your theory of being super realistic I will not have earned 89M yet because in this zone per day we have 100 EMS, around 10 Wild Fires, 1 or 2 Structural Fires, 3 or 4 car accidents...
And this not pay almost anything...