Hate to be the one advocating maximum realism, but anyways. In real curling one often can't know for sure who sits shot (or who's second, third or fourth for that matter). It's purely up to eye measurement as it here, apart for counting rocks lighting up. It sort of a part of the strategy determining what order the rocks are lying, as you can be pretty sure, but still be wrong.
In real curling the measuring stick (when needed) will only be brought out after the last shot of the end to determent the score of that end.