Quoted from Miguel351:
62" / 3(pieces as a result of two cuts) = 20.67
20.67 - (.1875 x 2) = 20.29 (.1875 is the thickness of the cutting disc)
The last .04" would be sanded for finishing.
The math is right, I was just taking a wild guess as to what the starting size would've been way back when they started making the special type of plywood used for playfields.
I'm a math teacher, not a carpenter(though I do pretend sometimes, ending in frustration more often than not), so maybe I'm not understanding you, but I think you overcounted the cuts. If the disc is only .1875, you wouldn't multiply that x 2 for each of the three 20.67" section(therefore giving you 6 total bladewidths). You would simply subtract (.1875 x 2) from the original 62" to account for the two cuts.
Anyway, I always thought they were that width because, well, it just feels nice. I'm not a big guy, but about average (5'9"), and standard width fits my stance better than even a widebody pin.