Terrain is only one factor that I use when building my random encounter tables. Technically, both the farmland surrounding a town and the wild open steppe are classified as plains.
The types of encounters between these types of areas are going to vary quite a bit. The farmland would consist of small animals, some herd animals, perhaps a wandering bear, and (wonder of wonders) lots of farmers! Untamed steppe is also likely to have herd animals and other wild creatures, but also more likely to contain genuine monsters.
Time of day or night is another factor to account for. Some predators are nocturnal and there may be a very good reason why some areas are scarier at night.
In settled lands, encounters on roads or in close proximity to them may differ greatly from those in the surrounding terrain even if the area is inhabited. On the road one is more likely to encounter passing merchants, farmers, pilgrims, soldiers, and so forth, at least during the day. Off road encounters may be very different even in the same region.
For these reasons, I usually customize encounter tables to the actual region being traversed instead of just by terrain type.