rerunning workflows with --cachedir sometimes repeats steps despite no changes to input

I have been looking at #71 for a description of how the `--cachedir` option works. As I understand various properties of the input files are encoded in a hash that is checked every time the workflow is rerun. If they match the previously calculated version is taken, otherwise it is recomputed (with all the steps that depend on that step).

I have tried to run a workflow a couple of times in succession without changing anything. However, some steps of the workflow are recomputed. I have also observed that the first step that is recomputed changes. I tried also to use the additional `--leave-tmpdir` parameter in combination and it seems to recompute less, however not consistently.

I am unsure whether I am using the feature incorrectly.

If this is unexpected behaviour, I am not too sure how to provide a reproducible example or what kind of diagnostics one would like to see to get to the bottom of this?

Thanks for the development!