chain_break_fraction result seems strange

**Description**
I see `chain_break_fraction` defined as `vectors['chain_break_fraction'] = broken_chains(target_sampleset, chains).mean(axis=1)[idxs]` where `broken_chains` is an array of T/F, so my expectation is that if a sample has n chains that `chain_break_fraction` can have values of `[i/n for i in range(n+1)]`.

In the example below I have 5 chains, so I'd understand values [0.0, 0.2, 0.4, 0.6, 0.8, 1.0] , but where does 0.09090909 come from?

```
SampleSet(rec.array([([1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0], -2., 420, 0.09090909),
([0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0], -2., 475, 0.09090909),
([1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1], -2., 2, 0. ),
([1, 1, 1, 0, 1, 0, 1, 0, 1, 1, 1], -2., 6, 0. ),
([1, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1], -2., 1, 0. ),
([1, 0, 0, 1, 1, 1, 0, 0, 1, 1, 0], -2., 12, 0. ),
([0, 0, 0, 1, 1, 1, 0, 0, 1, 1, 0], -2., 16, 0. ),
([1, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0], -2., 4, 0. ),
([1, 0, 0, 1, 1, 0, 0, 0, 1, 1, 0], -2., 14, 0. ),
([0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0], -2., 7, 0. ),
([0, 0, 0, 1, 1, 0, 0, 0, 1, 1, 0], -2., 19, 0. ),
([1, 0, 0, 1, 0, 0, 0, 0, 1, 1, 0], -2., 2, 0. ),
([0, 0, 0, 0, 1, 0, 1, 0, 1, 1, 0], -2., 1, 0. ),
([0, 0, 0, 1, 0, 0, 0, 0, 1, 1, 0], -2., 8, 0. ),
([0, 0, 0, 0, 0, 1, 1, 0, 0, 1, 0], -2., 4, 0. ),
([0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 1], -2., 8, 0. ),
([1, 0, 0, 1, 1, 1, 0, 0, 1, 1, 0], -2., 1, 0.09090909)],
dtype=[('sample', 'i1', (11,)), ('energy', '