Metering with a fader nd?

So I put a screw-in 2-8 stop fader ND filter on my lens since I need a low shutter speed for time lapse and its too bright outside. Does anyone know why the picture appears darker than the metered reading? Its on +3 right now but it appears as if its at 0 (evenly metered).

What camera are you using, and what lens, and what filter?

Like most ND filters, variable NDs do a grand job of blocking visible light, but they let a lot of inra-red through. That IR isn't going to affect your image very much, because the filter in front of the sensor blocks most of it. However, your metering sensor doesn't have such a filter and is sensitive to IR. So a properly exposed image, according to the exposure sensor, will turn out to be underexposed. How bad this effect is varies hugely between different filter manufacturers.

This is where some chimping comes in handy...