My 8yr old child caught chickenpox after 35 days we started isolation for covid. how is that possible?

Some basics: Chickenpox is one of the most contagious viral illnesses. It can be spread in airborne droplets or direct contact with the germ. A person is contagious from a day before blisters start until the last blister is dry or 7 days later. Being in the same room with a case for 15 min is enough to catch it. Incubation is 7-21 days ,ave.14. I doubt isolation was total or it is not chickenpox.