ECC key size and signature size

My understanding of ECDSA signature length is that it depends on the key size. So for instance, if a "prime256v1" is used, the signature length will be 64 because (n/8)*2 and for "secp384r1" it will be 96. However, I don't understand why is it for "secp521r1" the signature length is 132 instead of 130?

An ECDSA signature consists of two integers that can range between 0 and $n$, where $n$ is the curve order.
For secp521r1, the curve order is just a shade under $2^{521}-1$, hence it requires 521 bits to express one of those integers, or 1042 to express two.