Element of surprise, basically.
Everyone is mentioning "I was a kid", but that's not it. I wasn't a kid anymore for Norman's gorilla or Lenora's rodent of doom. What happens is that you're playing the story with your faves and in a rather relaxing way, and these mons get you unprepared because they pack a bit more of punch than the other gyms in their region. Once you know what they can do, you know how to respond.
Like the other anons said, there's also the fact you don't have that many options for your team to stop her this early in the game. Yes, there's Geoudude and Onix, but since they were so common back in gen I, it's likely you're skipping them for something else.
This "depends on your team" experience is more common than anons think, and it can be a game breaker. Most of the time, people play through the story with what they see and like, without much strategy put into it. I've done replays of my games and have problems with a gym leader that I didn't have in another run. When I played HGSS, I caught what I liked and while Whitney didn't destroyed me like back in the old days, I still have a couple of turns of trouble because most of my team was male and honestly I never look at the gender of the mons I catch.
