Seriousness of charges isn’t necessarily the same as seriousness of crime.
And:
it sounds like them departing from sentencing guidelines for armed robbery happened for the same reason as them wanting to charge him as an adult - I.e. that his actions were worse than you would think just from reading the name of the charge.
Not every third degree felony is identical to every other third degree felony. There are ones where the circumstances of the case are going to warrant a longer sentence, and those are probably going to overlap with the ones where the circumstances of the case would warrant charging a juvenile as an adult.
And, the least serious category of felonies are going to have more "space" for more serious circumstances to exist, and so it would make sense how those have this anomalous thing exist with them, that doesn't exist with the more serious felonies where the charge better encompasses the full seriousness of the crime.