The column gives you the volume of the sample containing the given amount of proteins... To check this, calculate the volume*concentration, it should give 53µg.
I fully understand your query. The values 2.32 to 9.03 (in ug/ml) signify the concentration proteins of your test samples (in each respective sample). Further, it means that a total of 53ug of your protein was loaded onto the SDS PAGE and for western blotting. Eg: If 2.32 is the protein concentration of your sample, then 53/2.32 = 22.84ul of the sample would have been loaded. Hopefully, this helps you gain more clarity.