I actually have to cut mine this short otherwise filth builds up and drives me crazy. I think maybe from digging in dirt while landscaping and when on welding jobs wearing hot sweaty gloves all the time promotes bacteria growth so under the nails it's itchy and has a build up of skin and grunge. This guy's thumb nail looks crazy short because his cuticle has extended out over the nail. Also, he could be inbred from the looks of that thumb morphology.