How does the installer decide

[Grizzly]

Hi All

Tried to install 21.04 on a PC with 20.04.3 & Win7 it only offerd to erase 
(20.04) and install or "something else"

But when I tried to install 21.04 on a PC with 16.04.6 it offerd to install 
along side 16.04.6, Erase and install or "something else"

So why?

Given the problems (some) people have had with installing 21.04, I didn't want 
to lose what is a working system, (yet) in favour of 21.04