Every single test is different. They each measure a different level of HCG and in early pregnancy your HCG is supposed to at least double every 2-3 days. The most sensitive tests can start detecting around 20 HCG while some others can only detect 100 HCG or higher.. so that can be a difference as much as 1 week between those 2 tests. The digitals detect at 50 HCG and up and those for some reason commonly give false positives.
I personally tested positive on a Dollar Tree test around 11 days after ovulation and that day my HCG was only around a 20. It doesn't seem like it but when you look at pregnancy forums the Dollar Tree tests actually have a good reputation for being one of the most sensitive tests.
And don't forget, any line that shows up in 10 minutes and is colored is positive! My line was very, very faint but it was enough to cause me to want the blood test
Thank you all for your replies! With my son, the test was positive right away. With my daughter, I had the faint line. I am feeling some symptoms of pregnancy now and it is way past my monthly time, but I have used six tests and all have been negative... Time to call the doctor I guess!