I think Steve changed that in a more recent version. The amount of minerals you get is fixed and dependant on the type of body, but in the range that the body would usually get. For example for terrestrial worlds I normally get 100,000 tons of a mineral.
I'm pretty sure the amount you get isn't fixed but capped. What's the chance your team finds 4,253,011 Duranium, compares it to the current amount of 4,197,310 and finds out an additional 55,701 can be added to the body deposit? No, they will either find 7,000,000 or 2,000,000 and add the capped quantity of 100,000 tons or nothing at all. I have seen teams occasionally find a random number of new minerals, while other finds on the same body were exactly 100,000.
Anyways, I've just read the new post from Steve about geology teams in the next version 5.7. This will all change soon.