There is quite a large amount of data and procedural help on the spectrophotometry of carotenoids available. One such source is a lipids database in Japan, and another is a highly useful two-volume carotenoid handbook from the late 1990s, the name of which escapes me at the moment. I will keep my comments general at first. One, carotenoids may have considerable shifts in their wavelength maxima in different solvents, on the order of 20 nanometers or so IIRC (obviously depending on which solvents are involved). Two, a typical carotenoid spectrum has a three-finger appearance, with each finger being a local maximum. It is possible that some people make their measurements at one maximum and other people make theirs at a different maximum. Three, a measurement in a crude extract may require that one take into account the presence of other chromophores. In my case chlorophyll was one such pigment. Point three is merely my conjecture, and even if it is critical for one situation might easily be different in another.