Another question about teeth whitening. Does this make a difference?

I have some trays of teeth whitening gel and on the pack it says something like: “use once per day for 15 minutes for 5 days”.

Does it really matter if I use twice a day for 15 minutes for 2 or 3 days instead? I can’t imagine how this affects how the product works. Or is that just it? The whitening doesn’t actually work? And if they don’t work why do they sell them? Isn’t it false advertising?