On both platforms, OmniGraffle's built on top of a certain amount of text-handling code provided by Apple. I know there are things the Mac frameworks can do that aren't supported in the iOS text system, but I can't be positive if that explains this.
(I'm not an engineer, and other folks here have stronger Graffle-Fu than I do. We browse the forums as we're able, but can't always guarantee that someone from a particular team will see every post.)
If you contact the
support ninjas, we'll get you an answer, though. Thanks!