Steals in general confuse me. I've had a guy with 95+ SPD end up getting much fewer steals than a guy with 80 SPD, in their careers. And I've been told exactly what TJ said, speed is the big one there. Amongst many other factors of course.
Steals have just seemed inconsistent for me. But I've also never put in serious effort to track it. It's a category with smaller sample size per season. As in, the difference between a 94 PER shooter and an 84 PER shooter seems to work out the way it appears it would by seasons and, for scoring. Lots of points get scored.
But the difference between a 94 SPD defender, and an 84 SPD defender, doesn't often translate to the steal totals i expect.