Different exposures for same shot in Manula Mode?

I covered a charity hockey game between the company I work for and former NHL stars. I was using a D3s and shot in manual mode. Most of the action shots (using a 300mm f2.8 lens) appear to be OK, but souvenir shots of the two teams and presentation of the cheques have me puzzled. Every 2nd or 3rd photo (no fixed pattern) appears to be overexposed by about 1 stop (for those shots I used a 24-70mm 2.8). These were also shot in CL mode. AF-S, f4.5, 1/50 sec, 1250 ISO

The Exif info is identical for all the photos.

Under what circumstances would consecutive shots, shot in manual mode register a different exposure? (My bracketing was off)

Any help would be appreciated.

Thanks,
Joe

PS. I cannot post the pictures due to the agreement with my company.