Why do pleated blinds have to be 2 centimeters wider than the windows they are covering?

Question

I recently bought pleated blinds and just now realized that they are not wide enough. The instruction says they have to be at least 2 centimeters wider than the window (on each side) they are covering. They are missing 3 centimeters on each side.

What is the reason for that requirement?

The mounting system can be seen here:

Mounting system part 1:

Mounting system part 2:

Answer 1

It's probably not a requirement. It's probably so that less light is shooting past the blind through the gap between it and the window frame.

If it is a requirement it probably has to do with the mounting system, which we have no information about.