The % is IV, which is immutable. You can have a 3500CP critter and still improve it using stardust and candy but IV cannot be bought. I use the trainer analysis for a rough IV estimate, then if I get a 3* I get an accurate IV reading from Pokegenie. The higher the IV (%), the better to use in raids/battles (depending on opponents/moveset etc).
I'm still looking for a Hundo Flappy Fish after catching over 3000 of the buggers. I have a couple of 98% which I've turned into Garys...