I suspect it is a historical/cultural thing for the UK and commonwealth countries. After all caning in a BDSM context is known as "The English Vice" and British public schools used to award the cane in increments of six and 12. ("six of the best") The Royal Navy flogged sailors with a cat of nine tails in multiples of 12.

I suppose it's a moot point unless you are always told what you are going to get. I much prefer this form of play where the Domme tells you the number of strokes you have coming (and with what implement or implements) before she starts rather than not knowing how long it will go on.

Whichever method you use, I've found both 60 and 5 dozen feel about the same......and hurt A LOT!