The cylinder leak down test is humorously referred to as "the A&P mechanics guaranteed employment test" because leak down of 25% or more in aircraft is considered "fail" even though the engine runs fine and makes plenty of power.

Consider the makeup of the tester, air flows through an orifice that is .040" in diameter...that is an opening of 0.00125 square inches in a cylinder that can be anywhere from 30 to 70 cubic inches in volume.