For the US bombs, the arming spinner required a set number of revolutions, not a set rpm.
That means the arming is about the distance the bomb has traveled, not the time it has traveled.
TD have been generous with their bomb timing of two seconds. Three to four seconds may have been closer to a historical figure. It was an essential precaution to prevent bumps in flight or bombs hitting each other just after they have dropped causing an explosion.
The idea is that the bomb will never explode when it is close enough to the plane to do damage (i.e. 25m away!)
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