I had to replace the roller blind I had bought some years ago at Spotlight. Measured lowest edge - 1200mm. So went shopping for a 1200mm blind. Found one labelled 120 cm. When I tried to hang it, I found the lower edge was 1160mm. Even measuring the top including the fittings, it only made 1190. Fortunately the window itself was narrower still so worked. But not what it was sold as!
Welcome to the community @willffarm
A blind for a 1200 wide window would reasonably be about 10~20mm narrower, but that should be consistent from top to bottom.
The top mount needs to be narrower than the window by just a little since it is a fixed mount, but since the blind needs to clear what are very often non-square windows (modern craftsmanship standards?) the bottom weight is commonly a bit narrower yet, so as to clear from top to bottom as it is raised and lowered.
A few years back dimensions were often the physical product width but changed to the ‘hole they fit’ widths, or vice versa, product dependent. Not a pleasant discovery as common practices change over time.
As an outcome, many products admonish the buyer to only trust measuring the actual product in hand prior to building whatever it goes into, not the nominal size advertised on the box or spec sheet.
As @PhilT said, the measurements on packaging are an indication of the window frame they fit. If you are fitting a blind into the window frame then the side fixing brackets must be allowed for, making the blind narrower than the window frame into which you are fitting. The top fittings have to sit off the window frame to allow the axles to turn, hence they are off-set inwards making the blind plus fittings narrower than the width of the fittings at the very top where you screwed into the window frame.