Positive OPK twice in one month?

Hello Everyone! I am new to posting but am looking for some answers. I have been using OPKs for 3 cycles now. This is our 6th month TTC. I recieved my first " peak" ovulation day on cycle day 12. Out of curiosity I tested again on CD18 and it was negative. So I tested again today ( CD 19) and go a high fertility symbol?? I have been using the clear blue advanced tests, I am terrible at being able to read the others. Just curious why I would have a " high " result a week later.