Well, it happened to me. Twice. As far as I am concerned, that pretty much proves it. Particularly when she said to me "You've only flown 85,000 miles this year. This gentleman has flown 95,000." Hoping I may avoid that scenario as a 1K in 2012....
Many agents do believe this is the reason and will verbalize it but based on numerous other examples it is not what the computer system uses. In most cases it is the computer that makes the decision not the GA.
The fact the agent stated this does not really mean it was the real reason.