Ok, so I've always suspected it, i.e. had a theory, but the CPC945 manual (section 7.2) confirms it.
If a machine has 4 GBytes of memory installed, and say, 1 GByte of I/O Memory is mapped at 0x80000000 (2 GBytes) upwards, then the physical memory will still be fully accessible. It will respond to read requests in the region 0x0 thru 0x7fffffff (i.e. 0 thru 2 GBytes - 1) and to the region 0xC0000000 thru 0x140000000 (i.e. 3 GBytes thru 5 GBytes).
This will of course only work if the CPU can make requests in that range, i.e. has a large enough address bus. Hence there is an actual hole that shadows physical memory when installing 4 GBytes in a x86-based System.