Every birth control method has a failure rate, even for perfect use, never mind for average use.
If all else stays the same but the amount of sex goes up, the number of babies will also go up.
Are the 25yo couples who are having more sex making other changes to decrease likelihood of pregnancy? Realistically? Probably not. They probably use their birth control the same way they always have, since it previously worked fine for the amount of sex they were having before. They don't necessarily think, "Oh, I'm increasing the odds that I'll get hit by the failure rate," especially if the way they've been using it has been "successful" for years.