a) because of Vegas' audio heritage - 44.1 is the standard for audio CDs.

b) if you render to DV or MPEG-2 the render format bypasses the project settings and renders the audio at 48kHz sample rate (it doesn't convert to 44.1 and then back to 48).

c) If you had your audio set at 48 and imported CD audio for music, or SFX from a sample CD these will all be at 44.1. Many projects will end up with mismatched sample rates somewhere along the line, but Vegas takes care or this transparently.