I read this last night, though at that time I was using the Blackberry and abso-positively HATE using this site on the Blackberry. As well as I wanted to think about the subject before dropping my 2 cents in the bucket.
The pull up is a wholly different exercise compared to the deadlift, however it can be done with different grips, and can thus work different muscles in coordination with the main muscle group that is the focus(upper back/shoulder/lateral muscles). With a "close grip" the bicep muscles as well as the main muscle groups are worked. With a "wide grip" the forearms as well as triceps and biceps are included in the mix.
While this isn't the same exercise it properly illustrates the point that, while a pull up with any specific grip can be used and have some effect on different muscle groups, it still the same level of effectiveness in the core focus.
As for it allowing you to move more weight. Assuming you're doing the workout properly every time, your arms shouldn't be doing any lifting at all, they're only there for stability. If using a differing grip is adding to the weight you can move it is more likely accociated with properly stabalizing the load, or tightness in the muscles after so many reps, or improper form for a certain set.
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