First I have to say I don't completely buy into this front/rear confusion thing. In my house the '6' speaker sounds very clearly behind me, no question. I've actually had to suppress a duck reflex more than once.

Having said that, the reason 'phantom center' and 5.1 (or 7.1) sound different than single center front or single center rear is simply that your ears hear everything, in a physical sense. So although your mind may process the identical parts of the L&R or 6&7 speakers and convince you it's all one source, each ear gets to hear each speaker. So rather than a single sonic event passing your head and exciting each ear once, you get two similar sonic events passing your head and exciting each ear twice, once per event. Of course your wetware can deal with this and figures it out, but it' not the same.

This is why for most people a phantom center is never as convincing as a real center.