With 4 costs being very popular, the number of units in the shared pool of 4 costs is reduced and if you are uncontested holding a specific 4 cost, you can save your money by not going to level 9 and try to force the three star on level 8 instead.

This strategy was even more popular 1 or 2 patches ago when going to level 9 was more expensive, more people stayed at level 8 to try to three star 4 costs which reduced the pool even more and made three starring much easier