In principle each instance could keep a running total to be added to the global total.
Consider the purpose of the votes: it’s a popularity contest for ranking within a timeline. Does the community on instance X want their timeine rankings to be influenced by votes by instance Y (which could even be prone to shenanigans)? A tiny instance might want external votes, in which case it should ask other instances what their totals are. But if an instance reaches a certain amount of activity, then external votes are actually pollution that makes scores less accurate/relevant with respect to the views of that particular community. E.g. the solar punk instance does not benefit from votes of those on a right-wing instance like hexbear.