determining service wire size

[xara]

I noticed 100 amps requires 1 awg aluminum THW wire in table NEC 310.15(b)(16). however, 310.15(b)(7) (dwelling services and feeders) requires 2 gauge Aluminum wire for a 100 amp service. Why do these tables differ?

 

[augie47]

310.15(B)(7) takes into account the load diversity for dwelling units with the realization the conductor would only see the full load for a short period of time.
In other situations the load diversity would likely not justify the reduced conductor size.