You have done a Compression Test...not a Leak-Down test.
Now that you have deducted you have a low compression cylinder, you now do a leak-down Test on that cylinder and that test will tell you why it has failed the compression test...
Leak test is not a Compression test..it is a leak detection test used to diagnose the fault/cause of a low compression cylinder .
So, it is a follow-up test to pin-point why you have low compression.

Search 'Leak Down Test'

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A Dalton