AD620 in-amp reference pin
In an ideal in amp the equation you have is true. In a real in amp we have to deal with something called offset voltage. This will be the real difference between the inverting and non-inverting inputs, as opposed to the 0 found in an ideal in amp.

Looking at the datasheet we see that it has a worst case value of 125uV. Multiplying this by the gain of 381 gives us about 48mV. Your output is therefore completely within spec.