Look up
transition altitude on wikipedia. Basically each country has its own predetermined altitude. When an aircraft is taking off it sets it baromoter to the current reading given by ATC and the height of the airport above sea level. Once it has reached this transition level it will then set it to 29.92. All other aircraft do this at the same height so this so they are capable of flying at different heights relative to each other regardless of the height of the ground.
The UK is set to 6000 ft (I think) whereas the USA is 18,000 ft (Again I think)
Senario :- If someone was to fly across the White cliffs of Dover, would you expect the plane to drop so it kept the same height from the top of the cliff and the sea !!!!
Read wiki, it makes more sense that what I've just typed
Matt