Parity Signer currently creates a 11 words recovery phrase (parity legacy brain wallet). It'll be fixed/changed by novasamatech/parity-signer#154 but in the mean time users might have created such accounts. It'd be convenient for users to be able to recover it on Fether as well, to keep things consistent across parity products. We should therefore accept 11 words. Given the parity legacy dictionary, the security is still higher than with a 12 words BIP39 so there is no security concern here.