If I set up a polyobject door like in the attached file (in this case, a double door made from 2x32-wide doors mirroring each other) and set their front lines to type 8 (Polyobj_DoorSlide), and give the doors a speed of 24 and a travel distance of 32, they actually travel a bit further than 32 units. I'm not sure if it's 33 units, or just under, but it seems pretty close to 33.
It's easy enough to see if you load the attached map and type iddt at the map so that you can see the lines, and then use the doors. You can see that the lines of the doors travel beyond the end of the containing sector (which gives the door 32 units to move).
You can also notice it at the game screen because the retracted doors leave a slight lip, rather than stopping flush with the walls either side.
My guess is that the answer is "use 16 or 32 instead of 24" and, certainly, if I use 32, the door travels the correct number of units. However, I find 24 to be a useful speed. So, if it is fixable, that would be nice.