Congrats on the new bike. The rear hub motor is totally independent of the gears when it runs it's essentially like you're on a normal bike going downhill. The lowest gears won't get you anywhere, you'll just spin your legs. With that said, the motor has some pep to it so there's really no use for the lowest gears, at least with the 500W motor they sell here in the States. On flat ground, I almost always start with the front derailleur on 2 and back on 4 or so. I typically ride at assist level 4, which means the bike is going to try to maintain 16mph which also means once I'm up to speed I am usually on 6 or sometimes 7th gear on the read derailleur. On steep hills there are definitely times I've used some of the lowest gears because the motor needs more help.
Ok, so as for the motor not assisting all the way to 15. It's how it works. PAS is more tied to a target speed for the bike. Sometimes it will overshoot it too when you get going so you'll find yourself going, maybe 15mph in one of the non-top assist level then you'll settle down to 12. There seems to be some other logic involved with the cadence also, so really high cadence may tell the bike you don't need as much assist because you're moving along fine, whereas if you are not spinning your legs as much the bike may translate that as you having a hard time so more assist? I don't really know, just a guess but I have notice cadence seems to have at least a little effect.