It has to do with LH surging strongest while you sleep so waiting until after 12pm gives it a chance to concentrate in the urine. Ofcourse you can still get a pos with FMU, it just could be a pos from the previous days urine so it's possible it could be misleading about OV day. But really your surge will show up whether it's FMU or not.
I actually use mine at around 11am - then when OV is close 2x/day at 11am and at 11pm. I have had short surges that were pos in the morning and neg in the evening, so I want to be sure I don't miss it! Also I usually have a fade-in, so I know if there is no hint of a pos in the am I don't need to test in the pmxx
Edit: Unlike guppy, I always get pos OPKs in the morning. Except this past month where I had 2 days in a row of pos, morning and night. Unusual for me!