There's a bug with the 27th gem where if you fight cortex and then collect your 27th gem in "Stormy Ascent" you miss out on 1%. This might also be the case for getting your 27th gem in "Hog Wild" or "Fumbling in the dark", please fix this ^^.
Many players including me encounter this very same and specific issue !
People are stuck at 104% instead of 105% even if every collectible (gem, keys, every platinium relic..) is collected.
Please fix it, it's so ***** annoying, it's excluded for me to start this whole game / collecting over again avoiding to collect the gem of one of those 3 optional levels "Stormy Ascent" "Hog Wild" or "Fumbling in the dark" AFTER the Cortex fight.