Do you mean the canons for the characters I play or canons in general?
I'm just good at remembering information, I guess? Most of the series I remember canon for are TV shows or manga, so it's visual and verbal and that just helps keep the information in my mind longer. I also tend to keep all of the canons separate in my mind so I'm not tripping myself up with character names and things like that (although it helps that voice actors stick in my head and often I connect the actor to the character to the series.)
Have you ever had the urge to get into a canon just so you can RP someone from it and if so, which one?