8-3/4 brake gap/ standoff odd measurement

DoctorDiff said
You must insert BOTH axles to test the clearance between the housing end and snap ring. If the axles are made to the stock length, they will not seat unless an 1/8" shim is installed.
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What’s the reason they won’t seat just curious ? Imprint test I did said there’s clearance in the middle, axle seal has room.