Ok, I've been wondering about this for quite a while, so I finally sat down & counted/charted bars & chord changes. The two A-sections are consistent at 14 bars (8+6) and follow the same progression (at the same rate) each time through. The bridge is 10 bars (it's on track to be 8 until bar 7, when he opens up the phrasing & makes what would usually be a 2 bar phrase into a 4 bar phrase. That sort of expansion fits right in w/what had been going on in the A-sections.
The final C-section, is in two halves. the first is 8 bars, and it's in the second where things get a little slippery. Sounds like Marvin suddenly decides to add a few beats before getting into the "and the home..." line (not a goof, but a spontaneous "feel" thing). That causes this section to resolve on the 7th bar instead of the 6th, which would have been in keeping with the length of the two A-sections, 8+6.
So yeah, Marvins' phrases are variable (but then again, Marvin was a phreaking master of phrasing - I've put together a CD of all the officially released versions of the Bobby Scott material, and the differences are RADICAL from take to take, year to year), but there's a symmetry & consistency to the accompaniment that leads me to believe that the arrangement was fixed in advance - and that there was a live keyboardist on hand to cover that extra bar towards the end.
It's an amazing performance, & I've known a few people over the years who tear up at just the mention of it. But that's the kind of guy Marvin was. He could do that to you.