Both of the tools run the same algorithm on your data.
When you run "detect categories" tool, it creates clustering model with most parameters set to default values.
When you run clustering tool, you can change some of the parameters to be different and thus you will get different model.
Also, if segments in your data are not very different from each other, you can get different model each time you run the algorithm.
Tatyana Yakushev [PredixionSoftware.com]