It has to do with two things:
1) the lack of agreed-upon definitions of who is to be counted as "homosexual." Sexual orientation is not either-or but is pretty much the way Kinsey depicted it on his six-point scale from totally homsexual to totally heterosexual. Most of us fall close to one end or the other, but lots of gays are married or have had opposite-sex relations. It's hard to know whom to count.
2) It's hard to do this kind of research in a homophobic society. You need honest answers and don't always get them. The CDC does keep statistics. But, while they say that 1.4 percent of men are openly gay, they use an estimate of 4 percent when talking about MSM (men who have sex with men). Many studies put the numbers much higher