Unexpected recompilations

I keep seeing the following behavior:
1) A qbs build finishes successfully.
2) When re-running the same qbs command line, a (small) number of files get recompiled and/or relinked, ven though nothing at all has happened in between.
3) This can usually be observed several times in a row, until the situation "settles down".
Two possible reasons come to mind:
a) Step 1) forgets to process some artifacts.
b) Step 2) erroneously classifies some artifacts as outdated (time stamp problem?).
Fact 3) seems to point to reason a). It also indicates that to get to the bottom of the problem, one should do a tracing build right after the problem manifests itself (and record all the time stamps before that).