Why do i have low compression in only one cylinder on a 95 dodge ram with a 360 magnum? i have 100-105 in all cylinder but one which has 90

Here is a easy way to find out,its called a leakage test and here is how you can do it,bring that cylinder top dead center,you may have to remove that valve cover and make sure all valves are bottomed out.put 50 psi of air in the spark plug hole remove radiator cap air cleaner and oil fill,listen for the air comeing out.if you have air coming from your air cleaner you have a intake valve bad,do the same with the exhaust,if you have bubbles in your radiator you have a blown headgasket,if you hear alot of air comeing from your oil fill you have a problem with that piston,I hope this helps you,