Question for those who may know more than me.

I had a non-refundable ticket that was cancelled due to extraordinary circumstances. It's a complex situation that I won't get into, however, AC has begrudgingly assumed responsibility after months of back and forth. They have processed a credit for the base fare, and have agreed to also refund the taxes/fees portion for the segments not flown.

With respect to taxes and fees, I paid the following:
September 11th Security Fee - United States(AY) CAD 7.70
Transportation International/Domestic Tax - United States(US) CAD 29.10
Passenger Facility Charge - United States(XF) CAD 6.20

I just received a refund today, and it they ended up only processing $7.70 as part of the refund. I am likely not going to bother following this up with them, but am curious more than anything about why the other items have been withheld for a flight not flown. Any ideas?