You’re taking about data rates here, measured in bits per second.

Data caps have to do with the total amount of data you are allocated over a longer period of time. Usually per month. In the case of Comcast, it’s 1.5 TB/month.

If the customer exceeds that allotment during the month, they will be charged an additional “overage fee” per arbitrary unit, usually by the gigabyte.

It has nothing to do with the speed they advertise on a line, but rather a way to charge “heavy users” more.