Not commenting on whether OP is being overcharged or not, but the best time to figure out how those charges were calculated was at the time of check-in. Ask them how they came up with the number they did and to show you the break down. Asking after the fact is often difficult if not impossible.

Just one more note, unless the OP was erroneously overcharged, this is yet another good example of why one should not go for the "lowest" fare available. Live & learn.

LAX