Help with metering for an ND filter

Apart from occasional use of a Polariser I haven't used filters previously

I imagined that being TTL metering I would attach the filter as normal and get the 'right' exposure

However I have repeatedly found (D800) that it will not give me the right exposure and I have to dial in +3 stop (ND8)

I'm a bit puzzled about this and tried my polariser (1st time since owning the D800) and that too doesn't meter properly - I have used both matrix and centre metering

Any advice? What am I missing here?

Thanks in advance