You don’t have the choice of making your bike shorter to fit you better, only longer, how can that possibly work well?
What about folk with long legs, short torso, and vice versa?
Say the standover on a medium is fine, but it feels way too long in the TT. So you try the next size down, the ETT is spot on, but you would have to run the seatpost above maximum extension.
People don’t just come in one size.