My quick guess is that you're not taking into account how Graphene treats canonical signatures.

For reference, check xeroc's excellent post on transaction signing: https://steemit.com/steem/@xeroc/steem-transaction-signing-in-a-nutshell

Here's an extract:

Now, we are going to implement a loop that is crucial because the back-end only accepts canonical signatures and we have no way of knowing if the signature that is going to be produced will be canonical. That said we will derive a deterministic k parameter for the ECDSA signing and while generating this parameter will will add our loop counter to the digest before hashing. This results in a new deterministic k each round which will result in either a canonical or non-canonical signature.