@Bruce P.
Hey Bruce I'm speculating here, but I would guess those values are the design firms' ways of covering themselves for the CAPACITY of the supply system. In a nutshell, even though the actual loads (ie: Btu/hr) of the water heater, cooling, and heating system, are not anywhere close to that - my guess is that Title 24 dictates what the supply capacity (ie: the NG lines typically for the water heater and furnace) must be. In other words, the pipe sizes need to be large enough to support for instance a future 'on demand' (ie tank-less) water heater as an example. These on demand units use a lot more Btu/hr right then and there, vs using less but 'storing' the heat. So, you need higher capacity but tend to get better energy performance from those on demand units.
I'm also speculating, on the intent of the law/Title 24 but I'd bet those values are talking about the incoming supply lines into the house - for instance it seems reasonable to supply up to 200kBtu/hr for the overall NG capacity, perhaps more - given that it's a nominal additional cost when the ground is opened up for those lines - and it allows more flexibility for future work (ie: adding on to the house, or advances in technology like that tank-less water heater I mentioned).