No idea what is going on there, but if you look at the OF position by position, with 500 innings minimum, 2 of 19 LF are above 0, 5 of 21 RF are above 0, and 13 of 18 CF are above 0. Totalled up that makes 20 of 58 OFs with above 500 innings who are a positive defensive OF according to fangraphs.