Total Float for the last activity

Dear Planners,

I am preparing time schedule for a project, once I finished I had a look on the massive total float for all the activities, I found some activities have massive total float (activities have no successor), so I started linke those activities to another to alleviate the total float, it was good but suddenly something happened I could not understand it, the last activity in the schedule (project completion date) has got total float (it should be zero)

Could any one tell me what to do?

Regards,

Moutaz