I believe you're saying you see a difference between the Expansion results when its a difference between two OPE cases and an OPE/SUS case?

OPE 1 = Max Temp
OPE 2 = Min Temp
SUS = As-Installed
EXP 1 = OPE 1 - OPE 2 (Max dT)
EXP 2 = OPE 1 - SUS (Max T - Sustained)

If you're doing an Expansion case between two OPE cases, it's typically maximum temperature - minimum temperature conditions. This would be like a steam line shutting down during winter or something like that.

The normal Expansion case between an OPE and SUS case is typically the minimum or maximum temperature - as installed (sustained) condition.

By default Caesar II assumes an installed temperature of 70F. So yes, the results between these two expansion cases will potentially be very different.