As a practical matter the ONLY reason for the 90c table is for the derating calculations. All ordinary lugs and terminals are rated for 75c or 60c.
One is also permitted to use the full rating on branch conductors for the crowding calculation, #10 @ 40A derated 50% = 20A.
#12 @ 30A derated to 70% = 21A ... still above standard derating of 20A for branch circuit #12 conductors.
When, in a 3-phase system, the fourth conductor is carrying only the unbalanced current AND no significant harmonics are present -- one may omit that fourth conductor from the wire count for the purposes of derating a bundle.
Meaning that four-wire feeder cable counts as just three and is NOT derated for bundling.
