I find it helpful to remember that the altimeter indicates the height that you are at, above the place where the actual pressure in the air is the pressure that the mb subscale is indicating.
So if the subscale is set to 1003mb (say) and that is the pressure today at sea level, then you are above sea level by whatever the altimeter reads.
If you set it to 1013mb so that you can say you are at a particular flight level, it is now telling you how high you are above 1013mb. Which could be below sea-level, depending on what the actual pressure is today.
And remember
QFE = above airfield = "height"
QNH = above sea level = "altitude"
and "elevation" means the height of the airfield above sealevel (which doesn't change of course!)