They've all lost matches to world champions at Wrestlemania
Incorrect. I'll leave the question up here for a bit, so hopefully someone may answer it.
Again, it deals with Wrestlemania and title REIGNS. Think about the uniqueness of each of their reigns, and what had happened "in the past" at Wrestlemania.
I'm pretty sure I got it.
Each guy wasn't on the wrestlemania card the year previous to when they defended the belt.
Sarg wasn't on WM6.
Flair not on WM7.
Nash not on WM10.
Sid not on WM12.
HBK not on WM13.