Believe they use UZR which takes forever (some say years) to give a useable sample. The number you're seeing on the page next to moncada is his average of runs saved (or not) relative to the league I believe and then position adjusted to come up with WAR (could be wrong, it was hard to find just now). So Yolmer, even tho he's a lousy 2B so far, he gets more credit than say a RF who's just average.