It matters because giveaways are generally an individual statistic. If Weber is generally on the ice for more scoring chances then it can be because of other factors such as the offensive players he is playing with, his zone starts, etc. The reality is that when Subban is on the ice, he touches the puck more than any other player and therefore his turnovers become the direct reason for the scoring chance. Weber's turnovers may go up on the Habs but he was something like 175th in the league in that department last season so he won't come up that high.Until Weber's turnover count increases it cannot be argued that he is any more responsible for these scoring chances against than Paul Gaustad or Roman Josi. When Subban makes an individual play at the offensive blue line and loses the puck, the 4 other players were not really the reason for that play. I've see instances where his teammates were moving their feet and/or wide open and he still makes a dangerous individualistic play that leads to a turnover in (once again) a dangerous part of the ice.
The best response could be that Weber can't catch a pass because he has a hole in his stick, or that he's so slow that he gets burned out wide 500% more than Subban and so other teams get their scoring chances that way as a result of Weber's deficiencies. I don't see it.
Better defensively? Maybe overall. But in all honesty some of his decisions only take 2 seconds to end up in the back of the net. I'm sure there will be turnovers by Weber that lead to goals but I've watched both play and the manner in which the turnovers are generally different.
Subban does a spin-o-Rama behind our net, gets rubbed off the puck by a second man and the puck squeaks out lose to an open player in the slot. Weber has the puck behind our net, tries to make an outlet pass to Beaulieu. Beaulieu gets pressured and returns the puck to Weber. Weber flips it hard off the boards to clear the zone but the puck hits a stanchion, takes a goofy bounce on to Krejci's stick and he's left all alone in front of Price.
Both those plays happened and both players looked goofy but regardless of skill, there's a higher percentage play between the two and it happened too often.