OpK test question ! :-)

I have been using ovulation prediction kits for two months now and trying to see if anyone has experienced this... I have only used the kind where you pee into a cup and use dip sticks. But when I finally see the second line I see it for 5-7 days after the first time. I make sure that I use a timer (4 minutes) so I dont get a false positive. Why am I see the second line for so long ?TIA