IVs are uniformly distributed, IV totals are not.
Just like how if you roll 2 dice, each probability is equally likely, but the total of the two rolls are not, since there's more combinations that equal 7 than 12 for example.
Also the IV distribution for wild Pokemon is different to the IV distribution for Pokemon hatched from eggs. The average IVs for wild Pokemon is 50% while it's 80% for eggs.