When you 'tac' reload, (with bullets still in the magazine) those are now lost from your total supply. A lot of people, myself included like to keep a full mag and reload after every kill. Before we have never been punished for doing this as it is incredibly annoying and it is entirely possible to move bullets from one mag to another in real life. However, in the heat of battle spent mags are dropped and left on the ground and DICE have now decided to operate this method in the BF4 beta. (whether this remains in the full game remains to be seen) but this is only really left to war combat "simulators" The ARMA series and Operation Flashpoint you would expect to see it.
However, this does play into the hands of the teamwork aspect of which the BF series in famed for. It makes the Support class more vital at keeping people fully stocked with the bullets and in the fight. Another thing I have noticed with vehicles it that they too have a limited supply of ammunition. This will replenish over time, but you can fire off shells and rockets much faster than they will come back. Meaning that if your not careful, you will run out and have to wait several seconds before being able to fire again. This reduces the dominance tanks and especially Attack helicopters can have on a game.
So watch that ammo count, and don't waste those bullets, shells and rockets. because you never know when you will need them.