+/- contains a lot of garbage like SHG and EN goals, and those things over a large sample can skew +/- largely due to deployment and team quality. This is why the worst +/- defenseman is usually a top pair offensive guy on a bad team, because these players are always out with the goalie pulled and get tons of minuses from EN against, and they're always on the PP which gives bonus SHG minuses. These are a ton of minuses and they aren't what people are thinking of when they dump on a player like Leddy for being bad defensively because he's -42. If you took the worst 6 defensemen in the league by +/- this year I think you'd be pretty happy with your top 6 (Leddy, Goligoski, Petry, Keith, Ekman-Larsson, Skjei).
The same applies in this case and an 800 game career gives a lot of time for the +/- garbage to pile up. Kane doesn't kill penalties and Toews does, so Toews being on the ice for ~5 SHG a year over 10 years is an additional +30-40 that Kane will never get because he'd never kill penalties. Kane will always be on the powerplay, and that's probably conceding ~5 SHG a year for an aggregate -30 (granted Toews would be on the PP too and get some of these minuses too). The other thing is empty net goals which mostly comes in to play by reinforcing the plusses on a good team and doubling down the minuses on a bad team.
The Blackhawks had a 9 year stretch as one of the best teams in the league, which means they would be in a lot of games where the other team pulls the goalie. Toews would be on the ice in these shutdown situations, and any EN goals the team scores would be an additional plus for him. Again, over an 800 game career that is a whole lot of pluses for Toews simply for empty netters, and Kane would never get any of these because he wouldn't be used in these situations. In contrast, Kane would only be on the ice in EN situations if the team was down a goal, and he'd get a lot of minuses for being on the ice for an EN goal against. You can see the effect this year, Kane and Keith were consistently plus players their whole career, but now that the Blackhawks are on the downswing they're getting tons of minuses because they're on the ice for EN goals against all the time because they're out trying to tie the game with a minute left.