If this product is for a rough opening of 24 inches, how can the products's width be 23.5 inches?

Hi- I am sorry for the confusion regarding the measurements of this baseboard register. Unfortunately, we do not sell a baseboard that is 25 inches long. The outside dimensions of this item are 24 inches width x 4 1/2 inches height (up the wall) x 2 3/4 inches depth. The duct opening measurements are 21 inches width by 4 1/2 inches height. We have a 1 ½ inch wide flange on each end of the baseboard diffuser with screw holes for mounting so if the duct opening is 24 inches or larger, there is no way you could attach the diffuser to the floor with screws. Baseboard diffusers are screwed to the floor, around the duct opening. Baseboard diffusers do not fall within the same measuring process as drop in floor registers or sidewall/ceiling registers. Standard industry baseboard diffuser descriptions use the space between the baseboard cut outs as the primary measurement. I hope that helps!