The platform refers to the physical structure, not the platform numbers. Platforms are assigned numbers for operational purposes. For example, some stations will have an island platform which has a single number on one side, and the other side has two numbers (Like 6A,6B etc), but both platforms 6 and 7 are sharing the same physical structure.
KGP has a single, continuous platform that is over 1000m long. And since it is so long, it is possible to start multiple trains simultaneously from the same platform. So, to avoid confusion ( imagine two trains being announced on the same platform simultaneously), each portion of the platform is assigned a different number.