Statistically why do women still get pregnant if condoms are used "perfectly"?

I'm not so much asking about the effectiveness of condoms as I am about the statistics. Wikipedia tells us "the perfect use pregnancy rate of condoms is 2% per year" and many sites about condoms and contraception also say the same. That means condoms are about 98% effective when used "perfectly." I would like to understand what causes the other 2% of pregnancies (or the other 2% of women who still get pregnant). I would appreciate an educated answer rather than chat/random spam/etc please, thanks.