Not always 300 waiting List means there are total 300 people waiting for confirm seat.
Sometimes Berth multiplexing and RSA arrangement make Huge amount confirmations. Many a times Tatkal and Premium tatkal seats doesn't get filled up , they are also released for filling these Waiting Lists.
Thats why railways keeps a "Buffer limit" which is "Regret". If Regret status is shown then NO further booking is possible.
sometimes...
more... in SL extra coaches are also attached to fill up the Waiting listers. So if they give just 60-80 Waiting Lists , then there is huge possibility that many seats will remain vacant after chart preparation.
And above all , its a way of some profits like for every last moment cancellation gives 80-100 rupees profit to Railways.