When they are disingenuous and inconsistent with how many nominations merits being up for the public vote it damages the integrity of the show. It shows poor sportsmanship and quite clearly frustrates the viewers.
On the flip side of the coin it means that they get to choose who stays and who goes, extending the entertainment value (in there eyes) of who they think will contribute most.
It's a poor strategy because it negates the whole process which in turn is far more damaging than any one individuals continued participation and is most likely the area they need to be focusing on to halt the nosedive in viewing figures.
In essence, interfering in such a bold manner with the format in a negative manner will cost them more than the eviction of any housemate ever will. They're just not intelligent enough to understand this.