Hard to know about the capacity w/o having the two pans in front of you, but it may just be that the one with the smaller diameter is slightly deeper.

Lots of factors go into capacity in addition to diameter, among them: configuration of the sidewalls , height, whether or not the top and bottom diameters are the same, and whether or not "3-quarts" is three quarts or a conversion from a metric figure.

The only sure way to know is to fill the pan with water, then measure it. But, of course, you can't do that without first buying it. :>)

If everything else about the Calphalon seems right, then I'd go for it. Whoever you're gifting with it can always exchange it if it isn't exactly what they're looking for.