I am currently playing Pokemon Crystal for the first time, and it's a lot of fun! I've been playing for a while, but I know I'm still in the early game in the grand scheme of things. I just recently made it to Ecruteak city, and my Pokemon are all in the mid to late 20s level wise. I came back to Route 32 in search of a low level Bellsprout because I mistakenly taught my Weepinbell an unnecessary HM (I'm an idiot, I know), and I decided to just go ahead and train up a new one. For whatever reason, I was having a really hard time finding one, even though I originally caught two of them in the same location (one for the Onix trade). I'd been riding around in the tall grass for a pretty long time when, suddenly, the wild encounter music played a very different tune.
Next thing I knew, I was staring at a level 40 Entei, with my pitiful level 25 Fearow staring back. I honestly didn't know I could encounter it this early in the game, and I wasn't even sure what to do. I tried attacking it in the hopes of weakening it enough to catch it, but of course, it immediately fled before I landed a single hit. Still shocked and now a little sad, I returned to my search for a Bellsprout. Then, on the very next encounter, I heard the unique encounter music again! This time it was Raikou, and my mind was kind of blown. Sadly, the same result followed and Raikou fled before I could leave a scratch on it.
After all that, I went straight to my computer and tried Googling it to see if anybody had experienced the same thing, but I didn't find anything. Has this ever happened to anybody else? I feel like it has to be some sort of mechanic that I just couldn't manage to find information about when I was searching online. Thanks in advance to any who respond!
TL;DR - The preceding paragraphs just serve to explain where I was (in game) and what I was doing there, the only thing I need answered is the question in the title, thanks!