The AMSDOS ROM also uses part of the RAM for XDPB storage and a sector buffer.
By having The TPA is limited because the BIOS in ROMCCP, using the BDOS and firmware and having the screen at &c000-&ffff this gives jumpblocks are in main RAM. The result is a ~33KB TPA for a CPC system which is OK.
* Amstrad CPM2.2 uses Amstrad's "System" format. This is 40 tracks, 1 side, 9 sectors per track numbered &41-&49. Each sector holds 512Bytes. There are two reserved tracks, then the directory which has 64 entries, and then the data area.