A lot of them are, but my favorite example, Twitter, really illustrates that some are wildly different.
This is its icon in System Settings:
View attachment 150030
But look at the notification icon when someone 'likes' your tweet:
View attachment 150033
Yep, our old friend, a heart!
And this is the icon when someone retweets you:
View attachment 150032
Looking at either of those, you'd never guess they had anything to do with Twitter, based on its system icon. And those aren't the only icons Twitter has, just two I had handy.
When you do the math, considering the bazillion Android apps that exist, it's mind-boggling how many notification icons there are!