Why are credits so sparse and always in demand?

I gotta ask-is the amount you get greatly weighted towards just one amount? I do the one where you can get up to 5 mill, and 99% of the time I always wind up getting 1.25 mill. ONCE I got the 5 mill-which vanished 5 minutes later, but, still, I'm not asking to get less than 1.25 vs the minimum of 750K, but, it's weird that save once, that's the only amount I've ever gotten. Not 2 mill, or 2.5, or 3 mill, or even 4.