Why are "Perinnials" seperated as a category from shrubs that are flowering?

Question

I am just curious, I have seen nurseries divide up their plant lists by Trees, Shrubs, and then "Perennials" and groundcovers and then of course annuals and vines, etc.
But my confusion is why they seperate "Perenials" from flowering shrubs. Because many of the thing they usually have listed under "Perenials" are looking like flowering shrubs to me.
Same confusion with groundcovers

AND ALSO CANT "GROUNDCOVERS" BE CONSIDERED "PERENNIALS" TOO ?

WHAT IS THE DIFFERENCE BETWEEN THE TERM "GROUNDCOVER" AND "PERENNIAL"??
I DONT UNDERSTAND SINCE "PERENNIALS" COULD BE USED AS "GROUNDCOVERS" AS WELL.

BY WHAT MICKEY HAS SAID THAT MEANS THAT A BUTTERFLY SHRUB COULD ALSO BE CONSIDERED A PERENNIAL, SINCE I CUT MINE TO THE GROUND DURING WINTER

Answer 1

most perennials die completely to the ground each winter and regrow from the roots, whereas shrubs regrow from the branch tips.
shrubs form woody trunks and branches--perennials generally dont.
Shrubs are considered to be permanent landscape features, perennials can be moved easily and many perennials are not truly permanent, some last only 2, 3 or 4 years before they need replacing.
shrubs stay where you put them, perennials often spread by root or by self seeding.

some perennials do look like shrubs (peonies, butterfly bush, hibiscus etc.) but they will die right to the ground every winter in most climates.

shrubs provide a permanent backbone for the garden, even in the winter

Answer 2

When nurseries and garden centres etc split things up like that they are refering to perennials plants that are herbacious. This just means they die down over the winter and just the roots remain leaving no stems or leaves alive. Shrubs and ground cover plants etc are mostly perennials but have stems and trunks that stay alive above the ground over winter. I think they split them like this so you have a better idea of the type of plant you are going to get and what its going to do so you can use it in your garden the best way. Hope that helps.