Arturia is probably stating that the width is 88 hp because they want some airflow around the edges of the module layout. This is a perfectly valid way to keep heat buildup in the cab from becoming a problem, as overheating can damage components (caps especially) over time, alter module calibrations, screw with tuning stability, and the like.
-- Lugia
I did discover that there is a 1HP difference after filling in the top row of my Rackbrute 6U. On modular grid, the HP maps out to fit the 88HP perfectly but when I fill in my modules, sure enough, there is a 1HP gap at the end. I wonder how effective/efficient that will be for overheating? They could have easily just built some circular or rectangular vents into the side of the Rackbrute.
JB