Sounds like bubble butt. Standing straight up, have someone check you out from front, side profile, and the back. If the bottom of your kilt in the back isnt level with the bottom edge in the front, you've got bubblebutt.

Either that, or you are wearing the back where you are supposed to, but the front is pushed down below the belly (if you have a belly) and the measurements when taken from there, are inaccurate with respect to the rest. Thats when a UK 'beergut cut' would be helpful.

If it is just bubblebutt, you can try pushing down the waistband in the read just before you sit, to give you a couple extra inches. So long as your shirt is tucked in and long enough you avoid 'plumbers cleavage'. Just be sure to pull everything back in order when you stand.