Correct, the MBR is on the oldest OS - and all the dual/multiple boot instructions you find on the WWW including MS sites explain this and also recommend to install oldest OS first, next oldest second, etc.

Yes you CAN install OS's in any order you want but that leads to a lots of issues (misery as far as I'm concerned) and after the fact gymnastics (like needing utilities like easyBCD) to get the boots to work correctly.

The MBR being on the Win XP drive is also why you need to HIDE the other OS drives (physical or logical) from Windows XP (see instructions above) because it's the reason WIN XP wipes out the other OS's restore points.

Larry