well... there is a circular argument if ever I saw one:
Junior F/O's are paid less because they don't have the experience required to hold an ATPL, but experience means nothing because they are a higher quality pilot.
So... the higher quality pilot is paid less?
BUT.... once the "higher quality" pilot gets some experience, he/she will be paid more, therefore joining the ranks of less qualified pilots. and then they will have the experience which we didn't want them to have in the first place.