It's possible that the sending unit float could have been bent during the pump change if you did not change the entire assembly out.
Another possibility is that the float unit could have been tweaked in such a way that the delicate contact in the unit is not as good as before. This would mean a higher electrical resistance in the fuel gauge circuit and a higher resistance means the gauge will read lower than normal.
This can also happen if the electrical connectors at the tank are simply unplugged and then reconnected. The resistance reading in the fuel gauge circuit may be increased and the result will be the same as tweaking the float arm.
This is one reason why extreme care must be used when working near the sending units. Unfortunately, sometimes this happens even if you are careful and means either replacing the sender or tearing it back apart and analyzing where the problem is. It's either the sender or the wire connections.