A few years back I made this post and recall it happening:

"I'm fairly sure that I didn't imagine this. Back 2 or 3 years ago when I was playing Pro Wrestling, I believe I somehow skipped the number of opponents I faced overall. I swear I only faced 12 opponents when I finished the game. Several things this could be: Using King Slender - it should add 5 matches to the total number of matches, however, I believe that losing in a specific manner may somehow screw this up. I have multiple copies of Pro Wrestling as well, it may have been some sort of version difference. It's at least worth experimenting on, but it may have been some strange fluke?"

However, I have yet to ever duplicate or see this happen again. Thought I would mention it here in case anyone wanted to look into it.

It could have just been my imagination, but I remember thinking it didn't add up right.

The wikipedia article on the game mentions something similar - but wasn't specific as to "some versions".

"Single player mode consists of two parts. First, the player fights in matches against increasingly difficult CPU opponents. After winning five matches, the player fights King Slender, the Video Wrestling Association (VWA) Champion. If the player has selected King Slender for play, then he will face Giant Panther for the VWA Championship, though some versions of the game have a bug requiring King Slender to win more than the usual five matches before being granted the title shot."

There are actually two US release versions as per this site: https://nesdir.github.io/E6F08E93_USA.html

There may very well be the bug you describe.