I think the whole notion of customers weighing specs carefully and choosing one high end Android phone over the other is overblown. If the salesman really wants to sell you the EVO you'll probably end up with the EVO.
Of course some people do come in with their heart set on something they or a friend had so I'm sure the brand thing you're talking about is real. I don't think it's the reason for the magnitude of what's being claimed for it here. Intending to buy a Samsung phone only works if the store is selling Samsung phones and we saw a nice version of this math play out while the iphone was only on AT&T and at 17% US market share. It's a tribute to Samsung's marketing staff that pretty much every store has a bunch of Samsung phones at all price points in it.
In another post here I demonstrate even if demand were even for the two of them the absence of the HTC One at Verizon would result in a huge sales disparity.
There's an oft cited idea that I don't entirely agree with but seems useful here: the real customers for Android phones are the carriers.