Just curious, it seems to be so high in B/W2 T_T
The rate of Pokémon encounter is determined from a simple mathematical formula:
1 in (187.5 / ( x )) per step
Let x equal the a value which determines how rare the Pokémon is. The higher the encounter rate, the more common the Pokémon is.
Encounter type Encounter rate
Very common 10
Very rare 1.25
Check here for more info. -----> http://bulbapedia.bulbagarden.net/wiki/Tall_grass