His average was awful for his first few seasons, but for the past 8 or so years his average was incredible. And 8 years is longer than most great bowlers' careers at test level. I think McGrath is a somewhat comparable player, and whilst McGrath had better consistency throughout the entire span of his career (and better average as a result), I think at absolute peak 2-3 years Jimmy's returns were better.
It's comparable to Tendulkar who has a great test average of 53, but on paper that doesn't appear to be 'greatest test batsmen of all time', which he was (aside from Bradman but hard to compare going that far back). But given he played test cricket from 16 to 40 that's inevitable. Discounting his scores as a teenager and in his late 30s his average is much higher, and rightly puts him way in front of the likes of Williamson, Smith, Labuschange, Kallis, Lara etc who have similiar averages on paper but didn't have a 24 year test career.