Pure Barred Rocks are Extended Black at the e locus so they are E/E with Barring, Self Lavender are also Extended Black at the e locus so they are E/E non-sex linked barring(no barring), that hen can't possibly be their offspring unless the parents are Not pure...
The only possible way for that pullet to have been sired by both parents are the following.
The Barred Rock sire is actually a Barred Rock/Columbian Rock progeny(perhaps not F1 but a few generations back so he is still carrying wheaten hidden under extended black so he is E/eWh ), The Lavender Dame is actual also another product of Lavender/Wheaten cross(perhaps not F1 but a few generations back so she is still carrying wheaten hidden under extended black so he is E/eWh
Both Parents are E/eWh so the chances of getting a pure eWh/eWh were 25%..
Columbian Rocks.
View attachment 3028658
Wheaten Araucana
View attachment 3028663
That is the ONLY WAY, because if Any of her parents were pure(Pure Extended Black) she would have come out pure black