You are comparing apples and pomegranates. The 20 ton (44800 lb.) figure is the archaic axial crusher measurement (normally some 10-15% below SAAMI radial crusher figures - expressed today as "CUP") and the SAAMI 62000 psi number is a piezo transducer result. SAAMI MAP for .308 Win. is 52000 CUP. Essentially, all these numbers represent the same pressure - it's the measuring systems that differ.
As a sanity check, consider the .303 British at -
41440 lb. (18.5 tons) axial crusher,
45000 CUP SAAMI crusher MAP,
49000 psi SAAMI transducer MAP.