B767:
Double engine failure = RAT drops down and powers the flt ctrls portion of the Ctr Hyd Sys, so no need for an APU to power the elec driven Hyd pumps.
Lose both AC buses, then no elec driven Hyd pumps but still got the eng driven hyd pumps. Take away those eng driven Hyd pumps for some reason and you've got the RAT. If you start the APU you'd obviously get AC power, so you'd get your elec driven Hyd pumps back. If the APU generator wasn't working for some reason you'd get air from the APU to the air driven pumps of the Ctr Hyd Sys but it is not clear in the manual if this air is sufficient to power the flt ctrls (by then the RAT should be working but I'm just postulating a scenario where it's not). The APU would, through the ADP (air driven pump), power the Ctr Hyd Sys, which is enough to power the Hyd Driven Gen but this just powers Stby buses and Transfer buses, so doesn't have the Ooomph to power the elec Hyd pumps.
So, in summary, engs fail = RAT. Flash up the APU = elec Hyd pumps ok. No RAT and no APU gen = Ctr Hyd Sys off APU air but probably not man enough to power the Hyd pumps (I'd be interested to know if someone knows differently).