The polariser filter factor varies, depending on the setting of the filter on the lens. The camera meter sees the difference and adjusts the reading accordingly. This peculiar feature of polarisers makes it difficult to use them with a seperate exposure meter. Whether or not you can dial in 0.3 or 1/3 stop of a difference depends on the camera. If it has aperture priority auto, the shutter speed is likely to be continuously variable, and will therefore set the exact speed for the aperture you have set. The readout may show the closest marked speed. In manual, you are usually stuck with set speeds, but the aperture can be tweaked to 1/3 or 2/3 between settings. I hope that makes sense!