No, under ideal circumstances the pregnancy prevention success rate would always be 100%, hence the word ideal.
What we are confronted with when talking about pregnancy prevention success rates are numbers for “perfect use” and “typical use” with the former denoting how high the chance of impregnation is during the first year of contraception usage when all the rules of the method chosen are strictly adhered to and the latter describing the chances of impregnation during the first year when the rules of the method chosen are somehow not followed correctly.
So when it says “15 percent failure rate under typical use” (this is what Wikipedia comes up with for the male condom), it means that out of 100 women 15 got pregnant during the first year of “typical” condom use. When used perfectly, only 2 out of 100 women got pregnant during the first year.
Yes, they conduct actual studies. The “typical use” group includes all people using condoms, among them folks who use condoms improperly and/or inconsistently.
The “perfect use” group only consists of couple who supposedly don’t make any mistakes when using condoms.
“Typical use” can mean any number of things concerning condoms. The condom might be applied inccorectly or slip off during intercourse. It might have been ripped by a fingernail when opening the blister. Or there might already have been semen on the male’s shaft before the condom was put on. Or the couple simply had intercourse without a condom (yes, this is “typical use”, too). There are many reasons immaginable why condom usage gets less effective by the user’s fault.