I do not think there is any official cutoff! They have a pool of applicants and they usually want to restrict the number of people with similar demographic background!
Just so happens that there are several people from India who apply with similar profile. So to distinguish the applicants, GMAT is used as an attribute. Thus, an Indian necessarily needs to have higher GMAT scores wrt other Indians to distinguish him/her!
I dont think schools have GMAT cut off per se! GMAT is just an indicator of your academic abilities and some analytical and verbal skills (besides ur undergrad scores). As long as u have a decent score to highlight that you can cope up with the studies you are fine!
Again, this is what I understand based on talking to current/past students and reading the various newsgroups! I might be wrong! Hjort can chip in for expert comments!