I'd be willing to bet more than 40% of iOS devices are not multi-homed - iPod Touches and wifi-only iPads.

From http://arstechnica.com/apple/2012/08/apple-lifts-curtain-on-... :

> Apple has sold 46.5 million iPod touch units in the US between the device's introduction in 2007 and the second quarter of 2012. (That's in addition to the nearly 86 million iPhones and 34 million iPads sold during that time.)

iPod Touches alone are 28% of total iOS sales. If wifi-only iPads make up 60% of iPad sales (and I think they're probably higher), then that's 40%. Consider also that the turnover rate on phones is probably higher than the more focused devices.