Thanks to slomacuser, this YouTube link further confirms the original color of the Macintosh by Jerry Manock himself as being Pantone 453 (about 16 min. into it).
Most likely, there are two factors at work here:
1) The color represents the actual 1983 Pantone formulation (which could be different) before it was mixed with and diluted by the plastic formulation, not the color of the final product and,
2) More probably, due to the nature of these plastics to yellow, the original color of the keypad was likely more green and even though "brand new" it has still likely been exposed to temperature variations and certainly 25 years of aging which due to the nature of the chemical processes involved would have yellowed slightly even if not exposed to direct UV.
He states that they found very quickly that PMS was not an accurate enough system for specifying plastics so they went to a Munsell system. So this seems to confirm in part the discrepancy you found in the keypad sample. It is highly unlikely that we can ever truly know what the exact color the original Mac was without a time machine.