What ^ said. Almost every element of this game has weighted probabilities. In the case of Credit Heist, the range is 1.5 - 10 million, but the probability to get 2.5 million has been calculated at 85%.
I tracked my credit heist results over 36 attempts (18 Credit Heists). My results were as follows:
1.5mil - 2; 2.5mil - 31, 5mil - 2, 10mil - 1
5.56%, 86.11%, 5.56%, 2.78%
Lastly, if you think there is a flaw / bug in the system. I would recommend tracking your heists or whatever else it is that you feel isn't working as intended so you can present specific data. Chances are you've received 1 or 2 attempts with a different result, but misremember due to the overwhelming number of 2.5mil results.