Or that was just an estimate based on what Burroughs was doing before and after that time period.
Burroughs gave what you might call a 'definitive statement,' and he gives no real indication whatsoever that was just some "estimate based on what Burroughs was doing before and after that time period."
FWIW, while the projectors started to roll at 1:00, the features didn't start until 1:20. The intervening time was filled with the usual mix of previews, ads, and, IIRC, a cartoon. Then, as today, many people showed up late to skip the filler. 1:00-1:07 is therefore an odd time for the guy taking tickets to bury his head in the inventory.