Canvas size is not showing exact size as I put in my properties

An image is just a rectangular collection of pixels. These pixels don't actually have a physical size. An image only takes on a physical size when we assign a resolution to it and thus define how many pixels per inch (PPI) there should be.

It looks like you are using 72 pixels per centimetre. Just wanted to confirm that you shouldn't be using pixels per inch.

If you really want to nerd out...

1 inch = 2.54cm

20 inches = 20 x 2.54cm = 50.8cm

50.8cm x 72 pixels per cm = 3,657.6 pixels

Can't have a fraction of a pixel so 3,657.6 becomes 3,658 pixels

3,658 pixels / 72 pixels per cm = 50.8055556cm

50.8055556cm / 2.54 = 20.0021872 inches = 20.002 inches (to 3 decimal places)