I am a newbie of CAESAR II

I have a very basic question about Load Cases:

For example: as the default Load Cases recommended by CAESAR II

L1: W+P1+T1 (OPE)
L2: W+P1 (SUS)
L3: L1-L2 (EXP)

As I know, the Load Case 3 (EXP) is used to check Secondary Stress.

My question is: why L3 is not represented as:

L3: T1 (EXP)

In fact , the stresses in the report are different between L3: L1-L2 (EXP) and L3: T1 (EXP).

Would you please explain it to solve my puzzlement .