My code check shows the following:
100 amp AL SE requires 2 gauge
100 amp AL branch requires 1/0 gauge
Is this due to the expected length of the branch circuit?
My chart shows you can still use #2 al for branch circuits depending on type and temperature rating. 90*C
If this is a single family dwelling, you can use Table 310.15(B)(6). This table accounts for the diversity in the use of electricity in the home, and permits smaller size conductors for the installation of the service entrance conductors than would normally be found in Table 310.16. 2 AWG Al is permitted in 310.15(B)(6) for a 100 amp service.
Using the 90 degree column for the final sizing of conductors is generally not permitted (only in very specific instances, of which I have never seen). The 90 degree column is for ampacity adjustment, with the 75 or 60 degree column used to determine the size of the conductor based on the specifics of the circuit.
For branch circuits, using Table 310.16, a 1 AWG Al conductor is permitted, but 1 AWG is not normall stocked, and because of supply and demand it may actually cost more than the 1/0… hence in your Code Check they show 1/0.
The reason for the difference in permitted size conductors, is the service has been determined to have more “diversity” in it than a branch circuit will have.