There is no way that the "Centurion part" could have known the gun held blanks - the weight difference could have been down to different types of ammunition or simply having less than a full magazine.
To keep a weapon's performance characteristics as consistant as possible you will find that different ammunition types, even from different manufacturers, will have near-identical weights. A bullet is nearly all of a cartridge's weight and the reason you can tell the difference with blanks is because there's no bullets. The gun will feel almost unloaded. Any sort of ammunition that weighed remotely the same amount as a blank round would alter the performance characteristics of the weapon severely, to the point it would basically seem like firing a different gun. It would also cause an extreme reduction in the weapon's stopping power. No one is going to produce and sell such a cartridge.
As to having a less than full clip: Again, the bullets are the main source of weight in a cartridge. The powder doesn't add enough to be noticible even if all the cartridges don't have any and all the cartridge casings together probably add up to a single bullet. If Zoe had any possible way of knowing there was more than one round in the magazine, she would have been able to deduce they were blanks. (And beyond a certain number of non-blank rounds there would be a perceptible change in balance.)
