Congrats in 1st place for your offer!
The starting dates are normally configured like this to constitute "classes" of Business Analysts and Associates, that start all together, have performance evaluations at the same time, etc.
However, this is the general case, given also that, as you commented, some people are still at school. There are many exceptions -in my year in Mckinsey, 3 of them- and you should just schedule a call with HR and expose the case.
Good luck!