Probably lost 1 of 3 phases. You're quite right in that the decision of what phase a circuit is on has a lot to do with business, and hopefully no major repurposing of the space without rewiring the space has occurred. For lighting, you'd want 1/3 of fixtures per room to go out, not 1/3 of your rooms in their entirety. For appliances and receptacles, you'd rather lose a whole function (the kitchen) than be able to cook but not do dishes, with every function trying to figure out oddball workarounds.