Note the similarity between PIT values and bits. If you have a good model of a data set and use it to encode the data, then the bit string you get out will be nearly random (frequency of 1s=50%, freq. 10s=25%, etc.) Analogously when you have a good model then the PIT values should be uniform on [0,1]. A tendency of the PITs to clump up in one section of the unit interval corresponds to a randomness deficiency in a bit string.