I don't understand ovulation question please

I brought two different brands of opk one forelife and the other babystart now I started testing from CD15 with forelife one and on CD 16/CD17 it look like I got a positive so I BD CD15/CD17 But on CD19 I tested again which the forelife brand and Bam two very very dark lines come up So I BD Last night which was CD19 but I use the other Brand babystart and the lines are becoming dark again why is this?