You both paid the 10% fee of 10 credits. Basically, you had 4100... WGT takes their 10% (10 cr) before you start, leaving you with 4090. Then add the 90 credits you won, and it comes up to 4180.
I had the same confusion when I started playing mpc's. The real winner in the deal is WGT because the players will just be moving credits around, minus WGT's 2 slices of the pie.