Is it normal for Pokemon to lose levels if you don't play a particular game for a long time?

Question

I haven't played my soulsilver in a few months, and when I opened it, I found out that my typhlosion had lost two levels, and my giratina seemed also to have this predicament.
I distinctly remember a guy giving me his giratina at level 81, but after two years it's at level
85. I don't battle that much, but after using as my primary sweeper ever since I obtained it,
It's minuscule gains somewhat startle me.

Answer 1

Pokemon do not gain or lose levels for no apparent reason, no matter how long you don't play. It must be a glitch or your imagination. I hope this helps!
-Jellohamster