You have multiple HD accounts under the same master seed. So the same backup can generate all these accounts, but mycelium does not allow to spend funds from multiple accounts in a single transaction (because the whole point of multiple accounts is to separate funds).

If you want to read up on the account structure, that is defined in BIP 44. (BIP 32 defines the general mechanism for key derivation in HD wallets, AND BIP 39 defines the word list backup).