I am sorry to hear of this situation for you. I suppose there could be a number of variables beginning with whether the other minors were in a vehicle at the time they were ticketed.
Not that the law distinguishes a passenger from a pedestrian but the District Attorney has that full discretion to base a plea agreement
upon the circumstances on a case by case basis.
Additionally, it could also be based upon your BAC. It does not have to do with the law enforcement officer however as they have no say in how one is sentenced. They only write the tickets
This is a Class C misdemeanor
and as such is punishable in the following manner
An underage person may be fined up to $500 and sentenced up to six months in jail.
So, I would suspect that in lieu of the jail time, your fine is being increased. I suggest you might want to ask your attorney why you are on probation for one year instead of 90 days. It could be that you are being given a "deferred" adjudication. That means that if you have not further incidents of any kind while on the probation, the guilty will be stricken from your record and you will not carry the misdemeanor.
But, of course, only your attorney can explain that to you , if that is the case.
Please let me know, if you have further questions in this regard.