I just checked my usage via the Mediacom site, and it shows a cap of 250GB/month. I have had Mediacom HSI for years, and have made no changes. I thought existing customers were grandfathered in with no usage caps.
You are grandfathered in currently. I don't see any internet changes to your account since August 1st. The number is just there to show what the allowance would be if you had your current level of service under the new plans.