Are there other explanations for slow tyre pressure loss?
Could be a faulty valve that only leaks under high pressure.  You won't be able to find this kind of thing by putting it under water because there's no way you'll get the tube up to 100 psi while it isn't on the rim with the tire on.  It could also be due to very very small holes in the tube, again which don't make any difference until the tire is at very high pressure.  You may be able to test this if you fill the tire up to full pressure and then leave it in the bath tub or other large sink.  you should be able to see if air is escaping from the valve this way. Not sure if you would be able to see air escaping from very small holes in the tube though as they might not be visible due to the tire/rim.