A “round” of bread comes from the way bread was baked before industrial baking where tins were first used. Bread was shaped prior to baking into a mound type pile and then baked. It generally came out “round” or circular. So a round was a whole loaf.
Also because bread was often placed directly on to hot coals the bottom would burn and become black. The top crust of the bread was then the most desirable and would be served to the higher ranking members of the tribe / clan / group / family. These were then given the nick name “the upper crust”.
The Wife has just said a round IS 2 slices of toast or a 2 slice sandwich. I’m not arguing..