Steve finds it hard to categorize Tony.
Oh, the man's a genius, he can't argue with that; but he never knows if he's going to walk into the kitchen and see Tony tucked up next to the coffee pot with a tablet in hand, muttering to himself and looking like he hasn't had enough sleep. Or if he'll even be dressed all the way, because Tony definitely seems to think that boxers and his various support devices count as clothes (and he's pretty sure Pepper's had to stop him from going out in public like that).
Or maybe Tony'll be heading down a hall, face faintly lined with pain - and Steve's learned that when he can see it, that means most people would probably be screaming from it - canes moving easily, and Steve will surreptitiously move just enough to not be in the way, because Tony hates it when people are more courteous when he's visibly acknowledging his disability.
(But he'd rather have people stand out of the way for his canes than move away and then look anywhere but at him when he's in his wheelchair.)
Sometimes, Steve goes down to Tony's lab, and maybe Tony's sitting in either his wheelchair or this ridiculous chair (which Tony once blithely told him cost two grand, and Steve choked, goggling at him, while Tony added that he would've paid ten for one that worked for him), but it doesn't make him less productive; the whole place is optimized for working from a chair, and with the helper-bots— Well, Tony mocks them, but Steve pays attention to what he has them doing, and has discovered that you can judge how well Tony's feeling by what his bots are doing.
Sometimes, Tony's in his wheelchair and it has an old and battered bag hooked over one arm; those are the days when he's hurting so much he's on medical stand-down, when the only way he'd get suited up is with an overdose of pain meds and literally being the last hero left alive on Earth. And he'll snarl and bite, words unfiltered, maybe even unprocessed, because his reaction to hurting is to make others hurt as much as he can; he's a self-admitted bastard like that.
Pepper never lets it get to her, Steve's learned not to take any of it to heart, and the others either ignore or avoid him. (Some of Tony's best inventions are thought up when he's in serious pain, because thinking is a distraction, lets him pretend he doesn't hurt and that the drugs are shit, though he never does more than draft plans, because he's not actually stupid. And that is a direct quote.)
Sometimes, he's in the Iron Man suit, and then you never know he's disabled at all, because Tony designed it so it doesn't articulate the way his own joints do ("Because that would just be asking to have something fail out on me, and defeat the point of it being supportive at all; you do realise that my joints aren't normal, right?"). He flies fast, fights hard, and when he gets slammed around, well, hey, it might hurt but he does his damndest to get back up. And when it fucks him up - because it can't not fuck him up, support and high-test metals only do so much, Tony explains once - he doesn't complain. (He complains more when he throws his ankle out by putting too much weight on it, or something equally absurd. Tony is the first to point out that "the human body is absolutely ridiculous, didn't anyone beta-test the design, what the hell.")
Steve grew up in a time when people were just starting to realize that your life didn't end because you were disabled; there's not a day, not an hour, spent in Tony's company that doesn't drive home just how true that is. Tony knows his limits, even if he pushes them (too much, sometimes), but one thing he doesn't know, will never know, is what "unable" means.
Steve respects Tony for that; admires him a bit, even, because he knows what it's like to keep going after life with both hands when everything's against you.
Steve may not know what to make of Tony - but he's okay with that.
It's part of Tony's charm.