The power gain I am pretty sure is from the protection gain.
If the character is unmodded than protection is the only stat that will go up when you hit upgrade
If you have mods that do a % increase, health, speed, offense, defense, those modded stats could go up slightly because if there was any gear at the newly completed gear level that could be affected by those stats they will now be included in the calculation.
Example
if you have basic speed of 152 at g10 and 12 of that speed is from gear items at g10. Speed mod sets would on give you 10% of 140
Once you upgrade to G11 speed mods would give you 10% of 152.
