UV Wavelength Definitions -
400nm > UVA > 315nm > UVB > 280nm > UVC > 100nm
Chlorine and Monochloramine absorbance - 259 nm (ϵ=484.61 mol−1 cm−1 ) and 331 nm (ϵ=95.7 l mol−1 cm−1)
Dichloramine Absorbance - 255 nm (ϵ=126.1 l mol−1cm−1) and 301 nm (ϵ=307.8 l mol−1 cm−1)
Nitrogen Trichloride Absorbance - 262 nm (ϵ=635.4 l mol−1 cm−1) and 344 nm (ϵ=315.0 l mol−1 cm−1)
Most UV sterilizers use low pressure mercury discharge bulbs where 80-85% of the UV is at 253.7nm and 5-10% is at 185nm. This puts most of the UV in the UVC range which is also at the wavelength of sterilization (pathogen destruction). Sunlight is mostly a UVA where one finds lower absorbance, and therefore less oxidation, of CCs. But, given how most pools are exposed to UVA throughout long periods of the daylight hours, even lower absorption of UV and oxidation of CCs has a noticeable effect.
My pool, which is open all year round and exposed to sunlight and UV for most of the daylight hours (with very intense UV in desert climate), almost never registers CCs of any kind. I think I once had one reading of CC's that was 0.5ppm and, honestly, it may have been a fluke. A covered pool will build up CCs over time.