If you are using Mocha3.2.1 and below only it is because the anchor point is also driving the null. Turn off the anchor point and your track and null should line up perfectly. InV4 we removedthe anchor keys for our AE Transform Data export.
RSP TURNS TO MOCHA PRO FOR THOR: RAGNAROK
MOCHA PLANAR TRACKING AND ROTO TOOLS FOR INVALUABLE TIME SAVING