Alarm clock and 9 minutes of sleep

When I woke up one morning, I was seriously puzzled by the question: “Why have all the alarm clocks that I've had over the past few years been postponing (snooze) the call for exactly 9 minutes ? Why not 5 or 10? Or, for example, to 12? ”This question interested me so much that I decided to google the topic a little and rummage around in the found.
Versions of Constant Origin
As a result of the search, there were quite a few different versions. I divided them into a couple of sections, which are presented below.

Funny version

• Five minutes is not enough. Ten minutes is a lot. Nine is not much different from ten, and looks appropriate.
• The number of seconds in nine minutes is pretty close to 2 ^ 9 = 512. The next power of two is too far.
• The frequency of the electrical network in America is 60Hz. Nine minutes is 60 * 6 * 9 = 32,400 cycles, which is pretty close to 2 ^ 15 = 32768.
• The reason also lies in mechanical alarm clocks, in which for some mechanical reason such an option was chosen.

Believable versions

• Psychologists have found out that “getting enough sleep”, which lasts more than 9 minutes, brings the person back to a deep sleep.
• The group of researchers confirmed that it is more convenient for them to finish 8-12 minutes (But why not 10 or 12?).
• The first copies of alarms with the snooze function, released in the late 50s, had a timeout equal to 9 minutes. Since then, so behaved.
• The nine-minute interval can be monitored by observing the value of one (last) digit.

UPD: According to the comments in the comments, it became clear that there are other options. I wrote about my alarm clocks - SE mobile phones (k550, and some other), iPhone