That's a really interesting question.
Pure saline solutions consist only of Na+ and Cl- ions in solution. As such, the pH of the saline SHOULD be neutral.
But it isn't. Saline solutions are typically marked as having a pH of anything between 4 and 7. Why?
I had to do some research on this, but it turns out that there are two possible causes:
1) Most people think that atmospheric carbon dioxide (CO2) is principally to blame. It dissolves in the saline, forming carbonic acid:CO2 + H20 --> HCO3- (equation left unbalanced)
2) Some authors also reason that the PVC packaging may increase the acidity of the solutions.