rich gave the concise answer. Now it's time for the long winded version
This is an abbreviated version of the answer to a similar question answered quite a few years ago (when these "multi-chemical-removing-tap-water-conditioners" became popular) by Dr. Anthony Calflo. Any mistakes are likely mine, and not from Dr. Calflo
The "stuff" (tap water conditioner, for ease of typing I'll call it stuff from now on) has X number of free molecules available to bind with whatever chemical compound it claims to remove. For simplicity's sake lets say 1 tsp has 1000 of these molecules to bind chlorine.
These molecules will float around until they can find a chlorine molecule to attach themselves to. When they find one, they bind themselves to it, turning it from chlorine, which is posionous to fish, to another less harmful chemical compound. When 1000 chlorine molecules have been found, the tsp. has done all it can do to "remove" chlorine from the water. To take out any more, you need to add more stuff.
Now lets consider that tsp of stuff has 1000 molecules that seek out chlorine, 1000 that seek out chloramine, 1000 that seek out ammonia, and 1000 that seek out copper. It doesn't matter when you add the tsp of stuff, these molecules will bounce around your tank water until they find a suitable chemical compound to attach themselves to. In theory, with enough water circulation this could be instant. In a more practical example, as long as your water is circulating as it should be, the stuff will bind to the chemical compounds faster than your fish can run them through their gills. The stuff is always able to remove quite a bit more unwanted chemicals than the volume of water listed for dosage. This doesn't mean you can use less, it means it will remove the chemicals faster and unused binding compounds either break down and leave the water as a gas, or are inert and remain in the water column until they find a chemical molecule they can attach to.
Are we sufficiently bored yet?