The short answer is this -
The
1.8m dish has a very short focal length
for its size.
The
typical .9 m dish is quite a bit longer.
Most
typical Ku band LNBF's are designed for a F/D ratio of 0.6 or 0.7 (0.6).
C-band dishes (this 1.8m is in that class) have a F/D around .3 or .4 (0.38).
So, for the LNBF to be able to "see" the entire surface of the dish...
... or to receive all the reflected energy right out to the edge...
...the
LNBF needs to be more "wide angle" , in camera terms.
There's plenty more info available, and specs to quote, and all, but that's the bare tale.
And all the talk about LNBs being optimized for prime focus or offset-feed dishes, amounts to nothing more than their reception angle.
F/D is focal length of the dish, divided by diameter of the dish.