Materials absorb different parts of the frequency spectrum with varying efficiency. Regular earplugs are made to simply stop as much sound as possible and hence

Materials absorb different parts of the frequency spectrum with varying efficiency. Regular earplugs are made to simply stop as much sound as possible and hence there's a lot of muffle at the top of the spectrum (that's easiest to achieve - cover your ears!). Concert earplugs are more carefully designed to absorb the entire spectrum as uniformly as possible.