To be honest, I'd say around 60% of the coasters in my count are like that. I often don't even recognise them. I'll remember one or two from each park, but that's usually about it.Yeah, I know I'm not on anywhere near your count, but even after the recent trip to the states, I've looked back like "Did I ride that? I really can't remember..."
That said, possibly more in the spirit of the topic, some rides are genuinely very forgettable. One that springs to mind (for some reason - maybe it's not that forgettable) is Hoosier Hurricane at Indiana Beach. I know I've ridden it, I know I've taken some nice pictures of it, I know it's location is lovely, but I can't remember a damn thing about how it rode. Not a single thing.