I wouldn't worry about it. Mine is the same way and the variance with mat on/ mat off is pretty consistent with two different turntables. Depending on the thickness of your mat, I would expect your readings are about .10 gram different. It may be anal, but I end up averaging the two readings to get where I want. Of course, there are those who say you should adjust by ear to finalize VTF anyway.
I actually think the mat off reading is probably more accurate for most records, unless most of your records are 180 or 200 gram pressings. Typically, the weighing platform, even on the best of the scales (I use the Canrong scale, also marketed for megabucks more as the Acoustech or Origin Live) is thicker than even a 200 gram pressing, giving you a slightly inaccurate reading compared to what a typical record is going to give you in terms of VTF.
Go for the average and don't sweat it.
I actually think the mat off reading is probably more accurate for most records, unless most of your records are 180 or 200 gram pressings. Typically, the weighing platform, even on the best of the scales (I use the Canrong scale, also marketed for megabucks more as the Acoustech or Origin Live) is thicker than even a 200 gram pressing, giving you a slightly inaccurate reading compared to what a typical record is going to give you in terms of VTF.
Go for the average and don't sweat it.