For these guys I just have slightly higher standards on what I consider good. I want guys with 2 pitches to have 80's or better across ratings to consider them bona fide studs I can rely on. Below that it seems they do get fairly inconsistent. Your guy is also a lefty which works against him. For lefties I want even higher ratings cause they get knocked around more than righties simply because there's more right handed hitters in the game.
Also, if you think that's bad, check out S55 of
this guy. He's gotten much more consistent over the seasons (after I traded him of course, lol) but he should still have something like a 2.50 career ERA rather than his 3.61 he's currently sporting.