One possibility is that it's probably a load balancing issue, and a characteristic of 3G networks (and 2G networks that have to co-exist with 3G). It's the same reason you'll find that in some cell phone support forums, people will complain that one phone will have full bars, while the exact same model phone sitting next to it will only have 1 or 2.
It's hard to explain in just a forum post, but basically, a network will predetermine what cell sites or channels a particular cell phone will have an affinity to. So, the network might prefer to have your phone "Camp" on say, odd numbered cell sectors or channels, while your wife's phone might be camping on the even numbers. This doesn't mean you'll ONLY be on certain sites and your wife's phone on others. If it needs to, the phone will pick any channel it can find. But, it will play favorites, because that's what the network is telling it to do.
The idea behind this is to make sure that a whole bunch of cell phones (who theoretically would make the same decisions all of the time) don't all decide to flood one channel just because it's the "best" one in the area.
Due to luck of the draw, your phone might be camping on a cell site whose 3G channel might not reach as far as the one your wife is on.
With 3G, people are going to start noticing some pretty interesting behaviors with the network. You're going to find that bars don't always mean a whole lot, and it's going to behave in ways that, from a network engineering standpoint is perfectly logical and normal, but to most cell phone users will seem downright weird. This is all a by-product of the technology shifting on AT&T from the TDMA-based GSM, to W-CDMA, which is a whole different way of running a network.
