There's a glitch in the Server, which is keeping track of previously owned cards.
The statistic Different Cards Owned "remembers" which cards you owned. If you trade away your last copy of a card, the next time you log in, the Server keeps track as if you still own it.
I realized this by accident, when I loaned a friend my playset of Glaceon GX. When I logged in the next time, the Different Cards Owned statistic was the same as before. And we tested (trading several cards back and forth) for another hour to replicate the problem. It was consistent.
If you add your Different Cards Owned to those Missing (show not owned), and come up with a different Total than 9,128, you've likely traded away the last copy of those "previously owned" cards (it doesn't subtract them). You'll need to replace those cards to get an accurate count (it won't count them a second time).
9,128 Different Cards to Own (the top possible, including those no longer obtainable).
P.S. Reported in the Game Bug section of the forum.
Edited by RobRatt, 06 May 2018 - 06:34 AM.