factory OC'ed, it means they tried to make the card faster than the others (aka ASUS vs Gigabyte) without actually telling you.
Or simply trying to fix the faulty card (card that had fault chip usually run at a lower specs - they increase voltage make it run faster means hotter and prone to failures)
I can't say for AMD being more friendly in OC'ed, it largely depend on the series, not who make it. The same company make the GPU chip for both AMD and Nvidia (TSMC).
Some card have lower rated specs for special reason, like the 5970 I'm using should have 15-20% more power than it is now but the AMD/ATI intentionally down clock it so it would run less noisy, and eats less electricity, but still giving the hardcore user to turn it back to their liking.