1. Your old T/P relief valve may have actually been stuck--clogged with meneral deposits, etc; and therefor it did not start to relieve the pressure at your 90 + expansion. The new valve of course operates normally. It's setting is 150 PSI; but that means fully open. Experience shows us that they may begin weeping at much lower pressure.

2. Your new heater probably has a better efficiency rating than the old one. It may also have a higher BTU. Heaters have changed considerably in many ways over the last 20 years.
So while your old heater did not react adverseley to your system problems ( 90PSI, closed system), your new heater performs as it was designed to do.