A rectangular domed roof

A ‘flat’ roof needs a fall on it to ‘shed water’.
However, a fall of 1 in 80 over 40m/2 would give about 250mm, but that is almost ‘flat’ for all practical purposes.
A typically two lane road has a cross-fall [camber] of about 1 in 40 for water-drainage, so for half of the 27m width 13.5/m4 that would be 338mm [ignoring the length’s fall] !
But why 153mm exactly ? Looks like 6" ?? That’s ~ 1/88 ?
How is the water removed from this roof’s edges ?

This amount of rise is more like a prestress for deflection than slope for water runoff.
Possibly?