Apparently it's because LH doesn't appear properly in the urine during the night at the start of the surge, so you need to clear out the night pee and the LH goes I to the urine by the second pee. Dunno how right that is to be honest but that's what I've heard. Plus if there is LH over night is can also give a false positive in the morning. So mid morning/early afternoon positives are more reliable
Last cycle I did one FMU and was negative, then peed again 30 mins later and it was a blazing positive