Something is wrong with your math here. No other reason than that. Not to worry too much, though, unless your net volume plus damping yields an effective volume that is way off the mark. A little off here and there isn't going to matter much, if at all. Cabinet size is often adjusted from the theoretical to the practical for aesthetic or other reasons, namely size or fit in a particular installation. Your ears will tell you... they are your best test instrument , and the receiver of good vibrations that make the musical experience... never lose sight of this fact while busying yourself with the math and science of it all .
As Zilch pointed out, actual testing of the finished system is the only way to see what you really, really ended up with. This includes the ear test
A couple of points:
Insulation, particularly fiberglass, adds virtual volume by converting the energy in the air from adiabatic (constant heat) to isothermal (constant temperature) which brings about a reduction in the speed of sound. loosely filling a box with insulation can easily increase the effective volume 20%; theoretically it can increase it by 40%. If you were shooting for 5CF, but your tests indicate 7.5CF, something is wrong here, either your physical dimensions are innacurate, or your test method to determine box tuning frequency is wrong. Insulation improves damping, which controls ringing and overshoot when the driver is fed a transient signal. R-11 is fine; I wouldn't use any less than this on the cabinet walls. This should add about 10% virtual volume, or as Drew has commented in the JBL enclosure guide, should about amount to about the same volume displaced by driver(s), bracing, and ports. A little thicker should add all that much more.
The same size port in a larger box will yield a lower tuned frequency. As Zilch pointed out, the design programs will get you close -if your physical dimensions and volume calculations are accurate. Always start long on the port as long as it is not too close to any obstruction i.e. a cabinet wall or bracing, and cut it to length to hit the actual tuned frequency desired. If your box volume is too big (or you think it is), fill the void with bricks, sandbags, or other solid, inert, incompressible material and give it a listen.
How exactly are you determining the tuned frequency of your box? You should be able to get close by feeding the woofer a sine wave at 100-200 watts, carefully observing cone movement as you sweep down through the frequencies below 60 Hz. At the box tuned frequency, port output is at maximum (as felt by your hand) while at the same time, the cone movement is at its' minimum (practically stops moving). The accurate method involves an oscilloscope and a resistor (PM me if you have a 'scope and want the setup instructions).
As you go below the tuned frequency, the cabinet ceases to load the woofer, and cone movement increases rapidly as you continue sweeping down. At this point, STOP! If you continue sweeping down, woofer excursion increases to the point where the voice coil can crash into the rear plate of the magnet and damage the coil. It can also be hard on the spider, surround, or even the cone itself. I've seen cones curl back near the suspension from over-excursion and even develop a circular fatigue line in the paper cone near the dust cap. This quickly leads to driver failure from severe over powering or driving below the ported box frequency. Just because the coil can handle the power doesn't mean you can't break the speaker...