The change in ASA speeds arose because they changed the ASA standard.
"The ASA standard underwent a major revision in 1960 with ASA PH2.5-1960, when the method to determine film speed was refined and previously applied safety factors against under-exposure were abandoned, effectively doubling the nominal speed of many black-and-white negative films. For example, an Ilford HP3 that has been rated at 200 ASA before 1960 was labeled 400 ASA afterwards without any change to the emulsion. Similar changes were applied to the DIN system with DIN 4512:1961-10 and the BS system with BS 1380:1963 in the following years."
So then what I said above is correct? (For once...haha)
EDIT: well I know Plus-X is 125 on the NEW standard so does that mean it WAS 64ASA before?