Because the application was for a 91-day stay, it could not be approved. Short-stay visas are strictly limited to 90 days. Anything longer requires a national visa, and these typically have an entirely separate application process.
The reason for choosing "Justification for the purpose and conditions of intended stay was not provided" is likely that the evidence submitted with the application wasn't sufficient to justify the requested period of stay, never mind that no evidence possibly could justify a 91-day short-stay visa. There simply isn't a better reason available on the multiple-choice refusal form.
Arriving before the permit starts is probably not a problem, but it might be. If the employment is expected to start on the first day of the permit's validity then it is highly unlikely that they would expect the worker to arrive on the same day.
"Information not reliable" is more worrisome, as it suggests that some evidence was too weak. This could mean that they thought it was fake, that they thought it was in the wrong form (e.g. a letter from the employer promising that the worker will leave after the contract, when the employer has no power to promise that), or that they wanted corroboration that wasn't provided, perhaps something along the lines of evidence of ties to a home outside the Schengen area. You will want to review the application with those possibilities in mind.
(Another possibility is that the job is some sort of scam, or they suspect it to be, and that the employer doesn't exist or they aren't convinced that the employer is legitimate. This seems rather less likely if the worker has already received a work permit, but it is worthy of mention nonetheless.)
