I have no idea why the builders would choose to select a 'false arch'.
Perhaps these are the prototypes of the free standing arch with which we are most familiar. I could reasonably imagine someone whose aesthetic levels were not compatible with straight lines introducing a curve. How the ancients understood load bearing and the consequence of dissipating loads through means of an arch has always left me wondering who understood the math way back then.
Of course, if this is true then the timeline of construction of such creations would bear closer scrutiny.