Bluetooth itself is a standard. There are several versions. For example my iphone 4s is bluetooth 2.1 It is quite possible that Ford's(Microsoft's) sync system can do things with some phone and not with others due to different versions.

Or as in any software there could be bugs but the iphone is for sure one of the models tested before they put this into production.

A few more years of this bull**** and then cars makes should start putting in just big screens on which your phone of choice will display what it needs to. No more car maker years behind electronics to hold you back.

add:
Just read a little about it and iphone 4s(I presume 5 too) is bluetooth 4.0 which is the latest version. Update your phone to the latest version; update sync to the latest version(can you do that yourself?) and all should be well. Sync being a Microsoft product it is still possible for them to add some special features that work only on Microsoft phones.