Table 310.16 is probably the most-used table in the NEC, and misreading it is one of the most common mistakes I see. The table gives you the allowable ampacity for copper and aluminum conductors in conduit or cable under specific conditions - but the conditions matter as much as the numbers.
The default assumption in 310.16 is: 75 degree C terminations, not more than 3 current-carrying conductors in a raceway, ambient temperature of 30 degrees C (86 degrees F). Change any of those conditions and you have to apply correction factors. Running 6 conductors in a conduit? Apply the 80% derating factor from Table 310.15(C)(1). Installing in an attic that hits 120 degrees F in summer? Apply the temperature correction factor from Table 310.15(B)(1). These factors stack - and when they do, your wire size can jump significantly.
The 75 degree C column is your working column for most termination scenarios. Breakers and equipment are typically rated for 75 degree C termination per UL, so even if you’re using 90 degree C wire (THHN), you size to the 75 degree C ampacity unless the equipment is specifically marked for 90 degree C terminations. This trips people up constantly - the wire can handle 90 degrees, but the connection can’t, so you use the lower rating.