Sneaky Apple Stuff
I am fairly convinced that the text layout engine in Safari on the Mac does some sneaky stuff to avoid orphans and generally make text look better. I say this, because identical layouts in identical fonts on the iPad often have nasty word wrap, and orphans, that do not appear on the Mac.
At first, I assumed it was a subtle difference in the way they handle kerning (like, maybe OSX works at a sub-pixel level finer than iOS). However, if that were the case then sometimes, the iPad would end up looking better than the Mac, and that never happens.
This makes developing a real pain, because you cannot test your text flow on the Mac and have confidence that it will look good on the iPad. I wish they would either update iOS to do the same great things OSX does, or at least give us a Developer Menu option to turn the magic off.