Thank you, I would bet on this. Examining an early digital clock with 9-minute snooze could probably confirm that the 9 minutes is implemented using some smaller number of components than any 10-minute system could be.
I also vaguely recall that such on such clocks, the alarm itself (if not silenced) would tend to run for 9 minutes before self-silencing. The same logic might apply: it's the longest finite interval that doesn't require some sort of multi-digit carry-logic.
I wouldn't be surprised if early circuit layouts even made it so that the wiring to toggle some [digit ± 1] triggering state was shortest/cheapest, in a circular arrangement, compared to any other potential offset. (I don't even think there was anything binary in the earliest digital-display clocks, rather some [10m, 6h, 24h] or even [10m, 6h, 12h, 2meridiem] cyclical counters.)
I also vaguely recall that such on such clocks, the alarm itself (if not silenced) would tend to run for 9 minutes before self-silencing. The same logic might apply: it's the longest finite interval that doesn't require some sort of multi-digit carry-logic.
I wouldn't be surprised if early circuit layouts even made it so that the wiring to toggle some [digit ± 1] triggering state was shortest/cheapest, in a circular arrangement, compared to any other potential offset. (I don't even think there was anything binary in the earliest digital-display clocks, rather some [10m, 6h, 24h] or even [10m, 6h, 12h, 2meridiem] cyclical counters.)