It depends on the event type. The event makes more of whatever spawn in that biome (IF IT EXISTS). For example our downtown doesn't normally have grass spawns. So when the grass event was on there weren't any more grass mons downtown because they don't normally spawn there.
But during the pink event, Porygon basically only spawned in the downtown and industrial areas. Porygon wouldn't normally spawn in the residential areas. So one had to go downtown for an increased chance of getting one.
During the grass event they increased the spawn rate of grass mons, they didn't add more grass spawn locations. This is KEY.