Yeah, was surprised at this too.. but then you have to consider that IRL only began in 1996. That plus half of the top drivers coming into their prime remained in the competition series CART plus some shifted back when CART relaunched as Champ Car) in 2004. The two series patches differences in 2008. Morso, the lure of NASCAR also claims tons of top drivers in their prime.
That said, holding the record, any of these records, is great. He's has 20 wins from 109 starts in the series (18% of his races). He's been really fortunate to have spent basically his entire career in one of the very top teams - always with a chance of winning.
This is why, when you look at F1, you see how amazing Schumacher really was. He had 91 wins from 249 starts (36% of his races).
Even Alonso has 21 wins in F1 (from 133 races = 16%) even though he's spent near on half his career in lower order teams with no chance of a win.