An accurate, definitive result is different than an early positive. If you get a positive result, it is accurate, even if you test before AF is due. It is testing for hCG, which is a hormone made in pregnancy, so to be positive that hormone needs to be in your system.
If you get a negative result, it is not necessarily accurate until about 14 or 15 days after ovulation. A negative test before then may just mean that not enough hCG has built up in your system to register on a test yet.
The reason is implantation. The hCG begins to slowly be produced when the fertilized egg implants and then it is usually 2 or 3 days later you can get a positive test. The tricky part is that implantation happens anywhere between 5-12dpo, so you may get a positive test sometime between 7 and 15dpo, but you won't usually know when! (Sometimes people have spotting just after implantation, for a day or two, which helps you know that you can probably test early.)
That's why some women get really early BFPs while others don't get one until AF is already late. And the sensitivity of the test isn't the reason. I hope I explained all that clearly!