I am not as knowledgeable as some folks here. but I believe that your young chicken is a cuckoo pattern not a barred pattern.
I borrowed this description from google
"Cuckoos are usually a darker, more smudgy, improper type barring. They also feather out normally. Barred birds are usually much more crisp and defined in barring, and a good non-production bred bird will have slow feathering. "
hatchery quality birds do not always hold true to the standard, while the cuckoo marans standard calls for one color leg doesn't mean that all the hatchery ones will be correct..
wait till she lays an egg the marans should lay a much darker brown than the plymouth rock.