It's probably because the police suggest they did not receive your licence details.
You should plead guilty when responding to the SJPN. If you don't want a personal hearing, in the "Mitigation" section you should explain that you complied with the conditions of the fixed penalty (by paying the penalty and submitting your licence details) and respectfully ask the court if they would consider sentencing you at the equivalent to the fixed penalty. They have guidance which suggests they should consider doing so:
"Where a penalty notice could not be offered or taken up for reasons unconnected with the offence itself, such as administrative difficulties outside the control of the offender, the starting point should be a fine equivalent to the amount of the penalty and no order of costs should be imposed. The offender should not be disadvantaged by the unavailability of the penalty notice in these circumstances."
