It's even weirder than that. For reference:
.mp1 = MPEG-1 Layer 1 Audio, used by no one and basically a historical footnote
.mp2 = MPEG-1 Layer 2 Audio, used by the radio broadcast industry for archival above 224 kbps and the audio format of VideoCDs
.mp3 = MPEG-1 Layer 3 Audio, the most familiar and widely used consumer audio format in the world
.mp4 = Advanced Audio Codec, AKA MPEG-2 Layer 7 Audio, later updated to MPEG-4 Layer 3 Audio
And that concludes today's installment of worthless trivia.