My understanding is that you can apply only a certain number of points from a stage race. So, for example, if you're in a smaller Cat 3 stage race with 49 starters and you win the Crit, you get 5th in the RR, and you get 3rd overall, you'd get 5 points from the crit, 3 points from the RR, and 10 points for the overall win. However, since you're limited to 15 points, you can only use 15 of the 18 points towards your upgrade.
This does a couple things. One is that if it's a longer stage race, like 3 days, and you end up doing well in a few races, you can't upgrade just on the one stage race. Say a less experienced (recently upgraded?) but super strong Cat 3 goes to a bigger (over 50 riders) stage race. He finishes at the back of the crit, wins the RR, wins the overall. He gets 10 for the RR win, 20 for the GC win, and technically he can upgrade with 30 points. However his experience level may not warrant his upgrade. He can only apply 15 points towards his upgrade. This forces said rider to do at least another stage race or a couple road races or something.
It also prevents a forcible upgrade from just one race. A good weekend may not justify a forced upgrade. I know one Cat 3 that upgraded to Cat 2 essentially on one stage race's results. He had few results before that. He ended up quitting racing one race later, returning to the sport a number of years later.
What you don't want to do is to make it easy for a strong but unskilled rider to upgrade quickly. I might be able to drive a car but that doesn't mean I can race one fluently.