The FULL values are based upon all frames on the SACCH multiframe, whether they have been transmitted from the base station or not.This means that if DTX DL has been used, the FULL values will be invalid for that period since they include bit-error measurements at periods when nothing has been sent tesulting in very high BER values.
On the other hand SUB values are based on the mandotry frames on the SACCH multiframe. These frames must always be transmitted. Thera are 2 frames fulfilling that criteria and this is the SACCH block and the block holding the SID frame. If the DTX DL is not in use, the SID frame will contain an ordinary speech frame and than this is included instead.