Do you have any debuffs that are lowering your STR, like from addictions? Still, that shouldn't be enough to bring down your carry weight by that much.
Have you used the console to change your carrying capacity at any point?
do you have lone wanderer? if yes did you take a companion that wasn't dog meat?
I'm pretty sure you don't get the benefits of your regular armor when wearing power armor, so that might be the discrepancy. Furthermore, it becomes weight that you have to account for...
This is probably silly, but have you tried to unequip everything and put it back on one piece at a time?...after each piece of armor check to see you're getting the 'pocketed' boost from your armor..