The egg and sperm did meet and tried to implant, so your body starts releasing HCG, but something goes wrong somewhere in the process. It's like a very early miscarriage -- you wouldn't even know about it unless you were testing before AF.
In my case, I had two days of faint positives, but then started spotting and over the next few days I started testing negative. AF was a couple of days later than normal, and was extremely heavy, but if I hadn't tested so early with such sensitive tests I would probably never have known I was pg at all.