The Uni has decided, for whatever reason, that they do not want to make offers below XXX, even though they have spare spots.
Reasons for them not wanting to lower the cutoff might include
Students below XXX might have a lower than acceptable risk of failure
For prestige reasons they want the cutoff to be above xxx.
They are hoping that other students (maybe above xx) will change prefs for round 2.
It is still possible they might lower the cutoff for round 2, but that is not very common