Ok I'm going to nerd here now, this thing grinds my math gears too much.
Consider two time slices, not checking for leap stuff : one between undisclosed dates of '80 and '90 and one between undisclosed dates of '90 and '99. The first slice has larger probability of being longer, that should be clear for everyone.
I do not believe this mistake is done by people who know the band, some less knowledgable will think Smith got in 1981 and exited in 1989 and get swayed by that factual error.
Also Smith exit, Gers enter is pinpointable down to a month, while Gers-only slice ending is a matter of debate (see discussion on reunion stuff in late 1998)