Teaching a pokemon a move but it loses the wrong move

[Honbuk]

So I wanted to teach my Lanturn Heal Bell and forget surf, but instead it forgot another move. At first I just thought that I accidentally pressed the wrong move to forget, but when I wanted to teach it volt switch and forget confuse ray, instead of forgetting confuse ray it forgot thunderwave, but I was 100% sure I pressed on confuse ray to forget.

 

Luckily I had enough TM's to get it right afterwards, but it was still annoying.