If you've cleared R1, MBB (and others) know that:
1. You can do the job at that level just fine.
2. You are smart, confident, analytical.
What they DON'T know yet (and will focus on in R2):
1. Will you be able to work effectively with a partner / sr. member. Also, cultural fit.
2. Are you consistent - was R1 success a fluke/good day?
3. Did the mistakes you make in R1 just a fluke/bad day?
So, R2 is just like R1 with some targeted focus areas based on how your R1 went. If they felt you were far too comfortable in R1, they might make R2 a bit more stuffy. In R1, everyone is rooting for you to succeed, but in R2 things do tend to get a bit more serious sometimes as you start meeting more Sr folks.