The Citi TYP card discount fare program is for the most part a scam because it counts the carrier's fuel portion of the fare as "taxes and fees." This results in a small discount on what could be a negligible "base fare."

JFK-LHR r/t (3451ow miles)
5/1 (AA 142) -5/8 (AA 101)

AA.com: $584 base fare + $217.30 in taxes and fees = $801.30 total
Spirit: $125 "base fare" + $655.30 in "taxes and fees" = $779.40 total

So, what really happened is that Citi gave you a 3.75% discount off the base fare.

Your rate will improve if you take REALLY short flights, but Citi manages to screw you even there:

LAN-DTW r/t (74ow miles)
5/1 (DL 3406) -5/8 (DL 3387)

delta.com: $238 base fare + $21.60 taxes and fees = $259.60 total
Spirit: $188.19 "base fare" + $38.20 taxes and fees = $226.39 total

This time, Citi gave you a 13.9% discount off the base fare. You got lucky, but still did not get the promised 15%.

Citi TYP card is not a complete scam - if you fly a lot, you will be able to offset that $125 annual fee. Say, six flights to London will do, or, if you prefer to stay domestic, you can rack up four of those 74-mile babies, and the savings will cover that annual fee.