Three or four reasons.

1. They're departing at different times. Different times of day are busier and the carriers will alter block time on that basis.
2. LAX. AS is building in extra time on the ground at LAX, where DL has an advantage due its terminal locations on the north side of the airport and less congested gate areas.
3. Different Ops. They're different operations. They schedule differently.
4. Aircraft. Don't know what aircraft are assigned, but some have faster cruise than others.