Yeah, I have found something in Pokemon Yellow that I think might be a glitch/bug, more likely to be a bug. When I first got my Pikachu (not exactly sure if I started before or after I delivered Oak's Parcel) but when I was training my Pokemon on Route 1, I found pokemon from level 2-7. Usually, it will be 2-4, like in Generations 3-4 (maybe 2 but I don't know because I have never played Silver, Gold or Crystal). So is it a bug, glitch or is it suppose to be like that?

its legitimately in the game.

you can find level 7 and 9 pidgeottos in virdean forest.