When multiple packages are passed to --print-variables, the output is printed in reverse order which is confusing to the user, e.g.:
$ pkg-config --print-variables udev systemd
I believe that the module list should be re-reversed before printing variables, or iterated from the end.
I didn't notice at the time, but this is also fixed by commit d6a1448, which moved the handling after the package list was reversed to the correct orientation.
I'm not sure how useful it is to print the variables from multiple packages, but I don't see a reason to remove that behavior right now.