On Pokémon White, when I was first in the Desert Resort, I encountered mainly Sandile and Darumaka. However, after training up my lead Pokémon, a Servine, from about level 20 or so to about level 30, I noticed that I was encountering many more Maracti and Sigilyph, and hardly any Sandile or Darumaka. When I first started training in the location, I rarely encountered Maracti or Sigilph, but at one point, when the level of my lead Pokémon was higher, I encountered six Maracti in a row, along with many Sigiliphs. Did my luck just take a turn for the better, or is there an explanation for this?
The only time your lead Pokemon has any effect to the Pokemon that appear in the wild, is if they have an ability that does so. Such as Pikachu with the Static ability. With him as your lead Pokemon, you have a higher chance to draw in Electric type Pokemon if they spawn in that area.
The level of your lead Pokemon would not have this type of effect, it's all RNG when it comes to encounters. So it was just coincidence that you started encountering different Pokemon after you leveled up Servine.
EDIT: If you were using a Repel, and the Pokemon you were encountering were HIGHER than level 30, then yes your lead Pokemon could effect what Spawns. If the other Pokemon you were encountering only spawn below level 30 and a repel is used, that could change what you encounter.