Why do some Queens express buses use Queens blvd while others use the L.I.E

Okay when taking express buses into Queens all bus lines from QV and CP (excluding downtown and super express buses) use Queens Blvd and the QM2,3,and 20 use Northern Blvd and the QM2 and 20 super express use Astoria Blvd and it really gets me annoyed. All the buses go past the the LIE and Queens blvd so why don't they let all the buses go on the LIE and does the QM2 and QM20 not allowed to use the GCP cause I see buses on it all the time.