The backup buffers are not part of the 20GB allocated to the buffer pool. They are outside the buffer pool.

Trying to find out how the 20GB of buffer pool is allocated is pointless, the backup buffers aren't there.

It could be that Windows was under memory pressure and so there was no free memory (outside of the 20GB allocated to SQL) for the backup buffers. Shouldn't have virtual address space fragmentation on 64 bit, so that's probably not it. I'd look for signs of external memory pressure.

btw what does this return?

SELECT @@Version