WinISD/Volume/Thiele-Small

Sealed enclosure, I presume?  Plug the manufacturer's recommended size into the project:  you'll note the change in QTC, as it goes down from the flat response of .707 as you increase the size.  You'll also notice a slight change in f3 as you increase the box size.  It goes a couple of Hz lower in the '40s when the box is bigger.  It would seem that the manufacturer is recommending the larger enclosure to increase the low end response.  Whereas the program gives you the size that will provide a flat response.  I'm using the latest version of Pro, and it gives you choices when you set up the project, of type of response you want.  You can pick flat .707 or a bass-ier response of .5-something.

There are some other programs you can plug parameters into also.  But the math used is all the same across the board, with all of them...even the ones the manufacturer uses.  You can look at it like this:  the recommendations are meant to cover installation for everybody, from the purist to the basshead.  So they necessarily want the average consumer to appreciate the lowest freqs that the sub will produce, without it getting too sloppy.  The result is some boominess along with those couple of extra low freqs, but the average listener isn't one who checks it out with calculations first as you are doing.  I would say if you are working on achieving SQ, you look at the graph on WinISD for the varying sizes and make a determination based on that.

Another thing, it wouldn't hurt to email tech support at Infinity and ask about that, also.