This is a common experience, but it arises because there were many slightly different versions of Fortran in actual use, taking advantage of the features of the

This is a common experience, but it arises because there were many slightly different versions of Fortran in actual use, taking advantage of the features of the many different compilers. But a program that stuck to the official standard generally never needed to be changed. (But I might be exaggerating about 1960. I don’t know when the standard settled down and the commitment to non-breaking changes was adopted. But it was a long time ago.)