Your filters block light to sone extend so that less light hits the film. Your non ttl light meter (or meter cell that does not have the filter in front of it) does not „know“ that the image that hits the film is darker than metered = underexposure.
In order to compensate you „fool“ your meter by lowering the ASA so that the metered light corresponds the light that passes through your filter and finally hits the film.
If you don‘t know the correct aperture/shutter speed combination when taking a picture, you have to use the light meter (with a filter, after lowering the ASA setting).