Grades after headhunter invitation to MBB?

Hi,

I recently received a call from a headhunter stating that an MBB (they obviously stated which MBB, but for anonymity sake) wants to meet me for an interview. Before the meeting with the MBB-firm I am scheduled to meet with the recruiting firm for further information, but stated with no impact on the MBB process. From what I've heard this is a reputable headhunting firm in my city and I currently work at a strategy boutique where I know my colleagues are fairly frequently approached from all MBB so this didn't suprise me. 

After the call the headhunter returned asking me to send a CV, which also makes sense as it is commonly used as a basis for discussion in interviews. 

However, after sending the CV, the headhunter came back once more saying that the MBB firm asked for my grades. This I found odd as it puts me in a position where it feels as if I'm applying for a job, which I'm not - they reached out to me! Also, I know that in terms of MBB-firm expectations my grades are definately too low. I was able to get my current job partly through contacts that could vouch for me, a very good CV from internships etc and great test results. 

So, to my questions:
- Is it common to ask for grades after inviting to an interview?
- Why do they suddenly ask for grades of a experienced hire, it was years ago I was in uni?
- Can the low grades result in them changing their mind and cancel the interview?