You would only know what a new starter might be offered, not what someone actually received after a period of time. I don't think the problem is organisations basing salaries on what other employers are offering (that's just market testing) but choosing to pay an individual person more or less based on what they personally managed to negotiate elsewhere. If I'm buddies with my department head and I get an extra $10k because of that, why should other employers replicate that when I move companies?
(Note : I don't really agree with this as it's relying on salary secrecy but I think that's the logic.)
(Note : I don't really agree with this as it's relying on salary secrecy but I think that's the logic.)