One way to interpret 「AはB」 is "something [that is] (B), which happens to be (A)".
Conversely, 「AがB」 can be interpreted as "(A) is [doing] something, which happens to be (B)".
One way to interpret 「AはB」 is "something [that is] (B), which happens to be (A)".
Conversely, 「AがB」 can be interpreted as "(A) is [doing] something, which happens to be (B)".