It's quite common for an altimeter to read less than zero on the ground with QFE set. QFE is the pressure at the highest point on the airfield's manoevering area. So unles you happen to be parked on the highest point, you will read less than zero.
Even so, 13mb difference between QFE and QNH, for an elevation of 327', sounds like a lot to me. 27ft/mb is, I think, the generally accepted approximation close to sea level, 13mb difference would suggest that they are using 26ft/mb. It is true that the TMA QNH may be higher than the local QNH, so that might be the reason. Why not pop into the control tower the next time you're there, and ask them? Then you can post the answer here, too!
FFF
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