Just wanted to share an explanation I gave to Xalt in a PM regarding finding Pokemon. 

"Here’s how it works. Common Pokémon are those that you can see just by going about the world. Uncommon Pokémon are Pokémon that require you to look around an area a bit more closely, asking around for information or fulfill a condition like head butting a tree. Rare Pokémon require more work as they typically require doing a challenge or finding unique locations by fulfilling certain conditions or by brute forcing your exploration. However, for both uncommon and rare Pokémon information is king so asking around for any information in towns and routes is valuable."

Hope this is helpful!