The people (Corey Sznajder, in particular, for zone exits) who compile unofficial microdata -- which isn't yet auto-tracked -- watch a **** ton of hockey to produce this type of information.
Yes, it is foolish for anyone to just look at a certain data point to arrive at a judgment of a player or team, just as dumb it would be to rely solely on your eyes since, though some are better-tuned observers than others, no individual can process everything that's simultaneously happening on the ice and without any bias. Hence the point of collecting and analyzing data.
Yeah, impressions often go hand-in-hand with the data when it comes to outliers -- players like Sidney Crosby and John Scott, for example, who are so obviously at the opposite extremes in terms on on-ice ability that it is easy for anybody watching the sport to quickly assess them as very good and very poor, respectively -- but it becomes much harder to distinguish among the bulk of the league's players who are distinguished by very marginal differences in various on-ice skills and overall talent. Beyond subjective preferences, our eyes aren't going to help us differentiate the second set of players much.
