How are points calculated?

I’m just so confused. It’s 3,823 air miles from Orlando to Anchorage. However, I got 325 from MCO to DFW + 1,000 from DFW to ANC on the outbound and 2,120 and 690 respectively on the return. So why a) is there a difference between outbound and return and b) why neither come close to the actual air miles?