I'm not an expert on this, but I get the impression that the signal is sent at quite a high level from the street cabinet, and during installation the engineer fits an attenuator behind your set-top box to bring the signal down to an acceptable level.
Presumably the level of attenuation is decided upon by reading the signal strength received at your set-top box, and so I think it's fair to say that one of the baffling figures displayed when you enter the engineering menus must indeed be signal strength !!