And the OS is specifically built to handle that. Layouts are't done in visual pixels, but in density-independent units. The layout engine itself is designed to accommodate different display sizes and aspect ratios. The entire OS is built around a capabilities system that is designed to make it run on devices that may or may not have a camera or trackpad or whatnot.
Android fragmentation is about the availability of API levels, not the variance in hardware.
> And the OS is specifically built to handle that.
Then it does a pretty crappy job. Why does the Play Store silently block apps from your device if this is the case? Why does every Android dev publish a list of supported devices? Because it's nowhere close to this simple, and can't be solved by improvements in the OS, which by the way don't reach the vast majority of users, hence the problem.
Well, my personal experience is that things break on different Android devices for various and sundry reasons despite the existence of dp. If apps run on the iOS simulator you rarely find issues on an actual device except performance issues. But you'd better test on the Note 2, Galaxy S2, Galaxy S3, Galaxy Tab, Nexus 7, HTC Desire C, LG Elite, ...
Android fragmentation is about the availability of API levels, not the variance in hardware.