Are we talking about Sarge's mustache again? Cool.

See, H1 Sarge has so much machismo that he shaves with a trench knife and the bile of his enemies. As one can imagine, this makes it difficult to accomplish the fine lip grooming that would be required to maintain the Hitler/John Waters hybrid that he appeared to have in older games. Due to this, the Halo 1 Sarge would have left that whole area alone and let it grow out naturally, because fuck it.

Halo 2/3 Sarge, however, probably has half of his personal effects dedicated to lady's BIC razors, so he would have no such difficulty.