It depends on how models reduce surface pressure to sea level pressure (the mean altitude of the Greenland ice sheet is some 2100 m above sea level, so there's a significant difference between surface pressure and sea level pressure over Greenland).
Some models assume an isothermal layer at the station temperature in this imaginary air column between surface and sea. But station (surface) temperatures on the Greenland ice sheet are often exceptionally low (owing to sharp inversions), resulting in a very cold (dense!) imaginary air column and an thus an exceptionally high sea level pressure.
I think it's anything but desirable to assume an isothermal layer in the entire air column - and most meteorologists agree with me. More and more models reduce surface pressure to sea level pressure differently these days.
