Why ambient temperature changes ampacity
Current limits are heat limits. A conductor is rated for the current at which its insulation reaches its temperature rating, and that depends on how much cooler the surroundings are. At 30°C ambient a 75°C conductor has 45°C of headroom to dissipate heat into. At 50°C ambient it has 25°C - barely half - so it carries substantially less.
Below the 30°C basis the factors exceed 1.00, which means a conductor in a cold location may legitimately carry more than its table value. That direction is rarely used in practice, because ambient can change and few designers want a circuit that depends on the weather.
Where this bites
Attics, rooftops (where solar gain can add substantially to the measured air temperature), boiler and mechanical rooms, and equipment enclosures. Rooftop conductor runs in particular have their own provisions worth knowing about.
Apply this factor and the bundling adjustment together - they multiply. The ampacity lookup does both and shows each step.