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The air in the tank will migrate thru the membrane of the tank causing a drop in pressure by about 1 PSI per year.   The way it was explained to me is this:   

Think of oxygen in the air (it makes up about 21% of the atmosphere) as having its own pressure that wants to be balanced.   So when you pump air into the tank, that tank is about 21% oxygen and 78 % Nitrogen with trace amounts of all the other gasses to fill in that other percentage.   Now the Air that is dissolved in the water in the boiler also has oxygen in it.  But that oxygen gets depleted as it works on rusting all the ferrous metals in the boiler system. Not a lot of rust happens because the oxygen gets used up pretty quickly.

Over a short time all the oxygen that is dissolved in the water will become a part of the iron oxides that make up the small amount of rust that forms.  (This is the reason you want oxygen barrier PEX tubing in the system so more oxygen does not migrate back into the system).   That is also the reason you do not want to keep adding fresh water with a fresh supply of dissolved oxygen to a closed system regularly. You want to keep that new oxygen to a minimum. 

As a result of the oxygen becoming chemically altered into other materials, the percentage of oxygen in the water becomes a lot less that 21% of the content of the air that is dissolved in the water  The air in the atmosphere is at 21% and the air that is in the water is at something lower than 21% so there is that unbalance that strives to be balanced.  If you have a barrier that allows the oxygen to migrate thru the membrane (like PEX tubing without the oxygen barrier) then that higher percentage oxygen on the outside will get into the water on the inside of the system.   That is what is happening with the membrane of the expansion tank.  Oxygen is migrating through the membrane.  After a year that oxygen migration will account for about 1 PSI pressure drop.

So you need to add air pressure every season in order to keep the tank at the proper pressure.    

This may not be the most scientific explanation but it gets the point across to us non-engineer, non-physicist, regular folk.  

It's recommended by the boiler and expansion tank manufacturers to check the expansion tank air pressure annually. I do this on all of my boiler maintenance jobs and I have found that the tanks normally lose a few PSI every year. I'm surprised if I find one that still has the same pressure in it as it did the year before.