You said an electrical engineer is going to comment… yes he is!
One of you made a statement that was so wrong, you should be ashamed of yourself. You said the ammeter showed how much the alternator was supplying. No, that is incorrect. It actually shows how much is flowing into or out of the battery. That is the important thing to measure, as you want to know if you still have proper charging. Otherwise, what would a negative reading mean? The alternator only puts out current in one direction, never sucks it back in.
As for the issue of an additional load having an effect on the battery charging, yes it does, but it is not a simple matter of the more load, the more charging. In the charging circuit, you have the alternator, the voltage regulator, battery and the electrical load. For a particular engine speed, there is a maximum current the alternator can produce. Additionally, the voltage regulator limits the alternator output, depending on the voltage it reads. Ideally, the regulator would read the battery terminal voltage, but for ease (lower cost) of manufacture, it actually reads the voltage at the alternator output.
For the particular case of the question, we have the alternator charging the battery, but the voltage regulator has sensed that there is enough voltage at its terminals, and is limiting the output. When a moderate load (the fan) is put on, the voltage regulator senses a voltage drop, and increases the alternator output to compensate. This increases the voltage, and hence the charging current at the battery. However, further increasing the electrical load (lights) now hits the maximum current the alternator can produce at this engine speed, and therefore has less current available for the battery, so the charging level drops, or even goes negative. More load will only make this condition worse.
While the act of putting on some accessories may possibly help to bring up the voltage regulator output, it is a tricky thing to find the maximum charging point. One would be best off in driving the vehicle, or having the engine run at some moderate speed for for a while.