It’s probably a combination of where they live and their genetics. Labradors have a distinctive undercoat so they tend to get drier skin in the winter months, which is when I’d guess most of them are shedding. In addition, also depending on the area, dirt from streets or pollution from vehicles can be an issue that can dull their coat appearance. Labradors might also darken with age- it seems like some breeds get lighter as they get older but not sure about labs specifically. Definitely some breed traits going on there though! Possibly line-breeding could come into play too? I’m no expert on labrador retrievers, sorry! There are lots of sites out there you might find useful if you are interested…