In short, yes.
In long:
It seems that there has been a lower amount of Hills&Sheep printed across most publishers. This could be due to the switch to the C2 artwork not long after H&S came out.
Most major expansions have seen multiple print runs from most publishers, which makes them more widely available (and yes, even the catapult had multiple print runs, at least in italy by Giochi Uniti).
But H&S had a shorter time before it became "old news", so the availability is reduced