Normal tach behavior?

[Nixon1]

I just had a quick question I had to ask about.. I noticed the other day while sitting at idle at a stop light that if I do something that draws power..for example, I hit my power window switch..I hit it up when the window was already all the way up so the motor didn't actually use the power, but it WAS drawn from the alternator. Now my alternator gauge reads very low. When I hit the button it dipped to the very bottom rung of the normal range. When it did that, I noticed my tachometer went up by about 50-100 rpms. The more powerful the accessory, the higher my tach spiked, but the engine rpms weren't actually increasing. Is this normal behavior, and could this be affecting the accuracy of the tach at upper rpms on the throttle?