I don’t have a Chromecast Ultra, but https://hevc.hhi.fraunhofer.de/HM-doc/_type_def_8h.html says:
enum Level::Name {
Level::NONE = 0, Level::LEVEL1 = 30, Level::LEVEL2 = 60, Level::LEVEL2_1 = 63,
Level::LEVEL3 = 90, Level::LEVEL3_1 = 93, Level::LEVEL4 = 120, Level::LEVEL4_1 = 123,
Level::LEVEL5 = 150, Level::LEVEL5_1 = 153, Level::LEVEL5_2 = 156, Level::LEVEL6 = 180,
Level::LEVEL6_1 = 183, Level::LEVEL6_2 = 186, Level::LEVEL8_5 = 255
}
So it seems as if levels are “compressed” into integers, and 120 is the H265 representation of level 4 (maybe in order to fit a larger number of sub-levels such as 4.1.2 in the future). For H264 the representation is different, and 42 means level 4.2.