mental vs. physical difficulty. the more in shape you are, the easier it is to get your head in control, on sketchy stuff. but even if you are a zen master like guy lacelle, if you are not strong enough to chuck a 6 foot horizontal dyno off an ice tool, you wont be doin musashi.
i also like how on wills gravsports website on the topo for howse of cards, he adopts and one-ups the Backes/House topo style for M16 by noting "NO BOLTS NO AID" probably the first time, such a big route was climbed on a rockies face not using any aid!