Because you or someone else installed both OSes on one partition, and it seems easier than starting over?
Just to see if it will work? (kind of, but not well). etc.
Thank you very much for the explanation! Stopped me from running yet another series of tests here to examine if maybe there were some advantage(s) to doing this. Always on the lookout here for workarounds, modifications and things we might not have thought of before, especially with the (little monsters) G4 Mac minis.

the general rule with the mini OS9 is that you should avoid all the hassle, disadvantages and required learning steps to have a dual boot situation on the internal HD - and simply use an external disk for OSX.
then holding alt or using the startup disk panel works as they should, and you do not need to turn on brain before sunday morning breakfast.
Well, I’ve had my Sunday morning breakfast and I should know the answer to this, but… is it necessary to first “Safe Boot” any external Tiger (or Leopard) install in order to prevent Spotlight and /or Time Machine from interfering with the internal OS 9 installation?
You know, the usual BTree errors?

https://macos9lives.com/smforum/index.php?topic=5108.msg36796#msg36796