Why two GPS on ship show slight different positions ?

1. satellite clock error –

This error is caused due to the error in the satellite’s clock w.r.t. GPS time. This is monitored by the ground based segments and any error in the satellite’s clock forms part of the 30 seconds’ navigational message.

1. Geometric dilution of precision (GDOP) –

The geometry of the satellite determines the angle of cut which in turns governs the quality of the position obtained. Wider the angular separation between the satellites, more accurate is the fix as the intersection will be almost at right angles.