LTC2470 reading jumps by 15

I'm using the LTC2470 to read the output of an analog hall effect sensor and it is working fine except that the reading jumps by steps of about 15 numerically as the sensor output changes (I assume 4 bits). Is there some reason why the 4 lowest bits would not be changing or somehow missing from transmission? I'm reading it with a PIC microcontroller and tried changing the SPI communication parameters which did not help.