Hi, in the old days, things had to live much longer. Some eproms were specified for 10 years, but a lot for 100 years. Compare that to nowadays electronics.
It also depends on how well the eprom was programmed, with the "speed" options of several programmers, the small eprom-capacitors may not have been charged to their full capacity but only a little above the "ok" limit. I can imagine that they loose their contents faster.
Pieter