That is true: the old laserdiscs of the SW films are always missing frames at reel changes. Usually it’s around 5 to 7 frames, but I’ve seen it go as high as 30 in a few extreme cases. It seems to be the oldest home video versions that are the worst offenders in this regard.
When assembling the theatrical stereo mixes for the Despecialized Editions, I always opted to use small pieces of the 1993 laserdisc soundtracks to fill in the holes. The alternatives would be either to let it drop to silence and then come back in, or to try to create a loop out of the existing audio. I found neither of these to be acceptable options, particularly in the larger gaps. Looping the audio risks being highly noticeable, and dropping to silence breaks the sense of immersion, so substituting a small piece of a different soundtrack yielded a much more seamless result, provided care was taken to match the level and equalization as closely as possible.
On rare occasions I also noticed examples of missing or repeated frames in the middle of a reel, but these were always much smaller and harder to pick out than the reel changes. I expect that Colson’s explanation is the correct one; the interpositive and/or internegative may have had small variations in frame count from the negative itself, and these would then make their way to release prints, which would deviate from home video versions that were derived from earlier generation sources.