It's the percentage of the chance of each pokemon spawning. The game doesn't make the Pokemons' chances compete with each other, rather rolling for each separate Pokemon to see if they are going to spawn there.
For example:
Deerling has a 90% chance of spawning while Cubchoo has a 10% chance of spawning. The game doesn't roll for them to compete with each other (for example Deerling spawning instead of Cubchoo if the game rolls that.)
The game will roll for each separately.
If both rolls are successful, both Pokemon will appear while if only one is successful, only one will appear. None of these two Pokemon might appear too if you have extremely bad luck (which is very unlikely.).
I know my answer was a bit messy and confusing, but I hope it helped clear things up :D
