It is a concession because "retro" is exactly that. They go back and make you whole for your 401K, your retirement, everything that compounds. Taking a signing bonus is a concession to the company. You have given them money to "use" free of interest for the whole time negotiations went on. It also gives them an incentive not to get the next contract done very fast either. If it will not cost them the retro they save money in the long run, hence no pressing need to get the next contract signed.