Research done by contributors to The Silph Road has concluded the Pokédex number of Pokémon is affecting their IV value. As mentioned before, IV values are the Individual Value of a Pokémon that determines its growth potential for Attack, Defense and HP.
The recent Appraise feature, introduced in the last update, uses this IV value to tell you if a Pokémon will be effective or not in battle.
The error causes Pokémon that appear earlier in the Pokédex, like Bulbasaur at number 1 in the Pokédex, to have a lower overall IV rating, while ones that appear much later, like Dratini at number 147, to have much a much better IV rating.
This bug only affects Pokémon caught in the wild. Unaffected Pokémon include those that hatch from eggs, ones caught while using Incense or ones from areas where one type of Pokémon spawns frequently known as Nests.
To put it simply – say you find a Squirtle (Pokédex number 4) out in the wild in an area you don’t usually see them there’s a very good chance it will have a bad IV rating and not worth spending powering up.
There is no work around for this issue, so it’s up to Niantic to either release a new patch for players or make changes to Pokémon spawns on their servers.