I know it's been stated before, and I know that B>Z, but WHY?!? Why does B cost almost double the price of Z domestically?

Yes, I know B is unrestricted and Z is restricted, but intenationally, B costs less than Z, as it should.

For p.s. flights I'm looking at in December JFK-SFO, round trip prices are:

B (BUA): $2390.91
Z (ZUANX): $1316.90

Heck, even M costs more than Z:

M (MUA): $1616.91

By the way, M is the fare I get when I choose "No Restrictions".