infinity sub incorrect port tuning?

the spec sheet for the infinity reference 1262w says that for a 2 ft^3 ported box, a 4x12.25 inch port is 30 Hz. every calculator i use and WinISD says that you need a 2.5 ft^3 box for that port to equal 30 Hz. what am i missing? here's the subwoofer spec sheet.

you arent taking into account the volume of the port and sub. usually the subs recommended airspace is the final after everything is accounted for. so to get 2 cubic feet of actual airspace, you would need to make a box that is closer to 2.5 to take into account the port volume and sub volume.

thanks for the clarification. so does this mean a ports tuning is configured to the gross volume? wouldn't the tuning change once you got to net volume?

the port has to be tuned to the net airspace, you have to predict the net volume and then try to make the actual volume fit your prediction.once you get close to what you want you will end up adding or subtracting port length to get your final tuning frequency but your average size port wont change the net volume so much, usually less then a 10th of a cubic foot if you just changed the length by a few inches. the net volume doesnt affect the tuning frequency as much as the port length will. this is how i use the calculators on this site, its not perfectly exact but its very close.

looks like i'm going with a box that's 2.275 ft^3 gross and a 4x15 inch port to end up with 2.144 ft^3 net and a tuning of 29.75 Hz.