I don't think I've really heard many people say Airlaw is a really easily understood topic, so just keep at it.
Your problem is that you're not really understanding the question, therefore the answer doesn't seem to make sense.
If the static pressure measured by the altimeter decreased from 1012mb to 1011mb, then yes, the indicated altitude would decrease. (Small point, pressure is now measured in hectopascals (hPa), not millibars, same thing just a new name, like Centigrade & Celcius).
However, what the question is actually saying, is that you are changing the altimeter sub scale setting from 1012mb to 1011mb, therefore you are moving the datum point from which your altitude is measured. As the point or level of 1011mb pressure is physically higher up than 1012mb, then if you measure your vertical position (which hasn't changed) from the new, higher datum, then it will be less.
You can try this quite easily on your next lesson, just sit in an aircraft on the ground, set the QNH on the altimeter subscale, it should then show airfield elevation (also your altitude). If you then move the datum on the subscale a mb or two either way, your indicated altitude will change accordingly.