Just thought this was widely know but I can't for the life of me remember how I learned this information in the first place, possibly some pet guide. The stat calculator on this site is spot on. But all Wild pets have lower health than their captured selves. Probably to not let pet battles drag on more than they already do when starting out.
This means that if you are hunting for a certain breed don't look at the health on the stat calculator, instead you need to check power and speed. One more thing though, you must remember if you are trying to capture a bird that they got double speed when you start the fight, just so you double-check that you found the correct breed.
Just one thing, earlier today I noticed one of my pets had 1 point difference in power to what the stat calculator said. But now I can't for the life of me remember which pet it was or the level. Im pretty sure it was in the 21-24 range. But I have made mistakes in the past so could possibly(probably) just be that I missed something.