travelnet and ZED fares

I'm a Delta retiree having my first experience with trying to use a ZED fare, wanting to fly on AA PLS-MIA one way only. I understand this has to be a paper ticket, and I can access the interline fare calculator...but here's what I don't get...the mileage is 578 miles PLS to MIA. That puts it in zone 2, which for a ZED medium fare shows $31 in the ZED chart. The interline fare calculator prices a one way MIA to PLS as $49.80 (which makes sense as the $31 plus taxes)...but when I try to price the reverse PLS to MIA no matter what date I put in, I get a total quote for a single person one way of $109.80. What am I missing? I can understand why the taxes would be higher coming into the US, but that much?? I am obviously missing something...can anybody enlighten me??? Bear with me, this is my first time trying to do this...