Non-optimal results

[photoneffect]

With the following values I get a less than optimal result:

bin:
0:50:22x18x16

items:
0:0:5:20.75x17x4,
1:0:5:19x12.5x6.5,
2:0:5:16.5x15x4,
3:0:5:16.5x15x4,
4:0:5:16.5x15x4

It only places 3 items in this bin. If I remove the last item it places 4 items in. Additionally, there is less wasted space in the second scenario. I could iterate over all the possible permutations and submit multiple calls, but I thought the algorithm did that itself. Is there a reason this doesn't return the most efficient result?