The doc is clear what happens but has anybody some use cases why this should happen?
Once the signature threshold is met if there are any leftover signatures then the transaction is regarded as having too many signatures which results in a failed transaction even if the remaining signatures are valid, i.e. for a transaction signed with N signatures, if the threshold is reached using K signatures then the transaction will fail if N > K.
https://www.stellar.org/developers/guides/concepts/multi-sig.html#thresholds