Hi bio61,
A change in temperature which is followed by a change in density does not change as well the pressure of the air?
Sea Level pressure depends on the mass of air over each square unit of area (sq. inch). If you change the temperature, you will have changed the height of the column of air.
How can it be possible that we ALWAYS find the SAME pressure at the SAME FL
That's all an Altimeter does. It may be graduated in feet - but it only measures pressure (Pressure Altitude) - you're real Altitude will change with air density.