I've realized that everytime I'm restarting a Pokemon x game there's always weedles and caterpies on the bottom left of the map, pansages/pours/sears near the the bottom right grass patches and my only pikachu encounters have always been near the pikachu trainer. So is there an actual encounter change? or am I just falling for some kind of confirmation bias?
Nope. All of the grass in Santalune Forest has the same rate. However I myself have thought why I always found the elemental monkeys in the top right. Most likely just a coincidence. Encounter rates shouldn't be any different unless the type of grass is different (eg. yellow flower grass and purple flower grass on route 7 riviere walk). I checked the Pokemon Database site of Santalune Forest and Bulbapedia to double check and couldn't find any difference in the grass