How is ComponentProportion calculated by fitgmdist

I run fitgmdist on some data (total of 40 instances/samples) and and get a two-component Gaussian Mixture Model with ComponentProportion containing an array of two values: 0.8927 and 0.1073. Using the cluster method on the resulting gmdistribution, and passing in the original 40-element array, it returns an array of cluster IDs, 36 of which have the value "1", and 4 of which have the value "2". My expectation was that ComponentProportion would have contained exactly [0.9, 1.0] to reflect the ratios of cluster IDs in the data being fit. But apparently that's not the case. Can anyone tell me where the actual ComponentProportion values come from? Thanks.