When receiving weather fax, it is important to have a very clear and good signal. The image is generated via a series of pitches. One pitch for white, another pitch for black. If the pitches are not distinct or there is a lot of background noise, your image will be distorted.
A relatively clear signal will produce an image like this. Few hiccups and a generally legible image of the weather systems at work around us.
Wind noise, conversations in the boat, or just bad reception will produce something like this. You can still make out the general context of the fax, but it is not as clear or legible as when you have good signal.
Now, bad reception and no time to calibrate your image as it prints onto your tablet will generate something like this. The image is offset, and it is barely legible. All you can really make out is that it's supposed to be a map!
Be sure to calibrate your setup and take advantage of the test run that NOAA sends out at the start of the weather fax transmission. If you get a clear image at the beginning, then all the subsequent faxes will be clear as well.