I had the same issue with PS a while back when trying to make kiss-cut stickers in a very exact size for a custom request. I was working in inches not centimeters but same thing, PS was adjusting either the height or width by a little bit. I chalked it up to some mathematical formula of width, height, pixels, and ppi that's over my head but makes sense that one can't specify ALL of those and have them all work out exactly, something's going to have be "shifted" a little to make it work mathematically.
I just tried to create a new document in PS (in centimeters) at 40 x 45, with the default resolution of 72ppi (pixels per inch). That produced a 40.01 cm x 45.01 cm / 72 ppi canvas.
Then I did 40 x 45 at 150 ppi. That produced 40 x 44.99 / 150 ppi. Same as you're getting.
Then I did 40 x 45 at 300 pp. That produced 40 x 45 / 299.999 ppi.
I also tried changing the resolution setting when creating a new document from pixels/inch to pixels/centimeter to match that I was setting the width & height in centimeters. When I put in 40 x 45 at 150pixels/cm for resolution, that produced exactly that - 40cm x 45cm at 150 ppcm. My brain can't think it centimeters so I used this calculator to see how 150ppcm compares to ppi.
It's a lot, more than the 300ppi often recommended here to create in.
I think there are mathematical restraints here but I would suggest changing your resolution to Pixels/Centimeter when creating your new document, so it's the same measurement unit as your height & width.
Then play around with the number in the Resolution box until you get an outcome that works for you on all fronts.
