I was almost fooled by the happy
[tar] Nothing to do: filename.tar is up to date.
message, but then better sense prevailed and I reran my build script after deleting the filename.tar file. The tar file created on the second run was bigger (and hence, different) than the former one.
But why did I suspect the Ant message at the first instance? Because, the previous run had failed with the following message:
BUILD FAILED
build.xml:51: Problem creating TAR: No space left on device
However, after freeing up disk space, I had expected Ant to remove the half-built tar file and recreate it.