Errors have been a decreasingly useful stat since the early 20th century. MLB has of late taken pains to regularize the calling of errors by Official Scorers, generally in the direction of calling things base hits that look like errors to most of us, to the point that I don't find it (or its close relative, Fielding Percentage) a useful stat at all.
I do see that, in roughly similar innings so far this year, Julien has about 20% fewer total chances than Polanco per 9 innings. Perhaps that reflects an unequal number of balls hit their respective way, or reflects a significant difference in range, or just reflects scorers' decisions on plays not made that take Julien off the hook by calling them base hits. I don't know. There are other measures of defense than this simple "range factor", and all of them have flaws. The eye test for most of us seems to be consistent with the range factor for the year, and that is where I choose to stop thinking on this topic. If the range factor is close to meaningful, then every couple of games, Polanco would make a play that turns into an out that Julien could not.