Hey,
the APIC registers are mapped to these physical addresses, so they are accessible on those. When the processor accesses these addresses, it will be read/written to the APIC registers instead of the physically available RAM. In the physical address space there could also be other devices mapped.
After all, you can't be sure that when you have 32MB of memory they will be physically available from 0x00000000 to 0x02000000. It could be mapped in different parts to different locations. See this nice page:
http://wiki.osdev.org/Detecting_Memory_(x86)
Greets