Most in the industry have long understood that pump motor nameplates can be very deceiving which often leads to incorrect sizing. It is bad enough that manufactures have chosen to have two classes of IG pumps, up rated vs full rated, but AG pumps also come in an "Special HP" (SPL) version which is basically a double up rated pump.
So what do all these terms mean?
The horsepower required to turn an impeller at a specific RPM is called brake HP (BHP). The required BHP changes along a pump's head curve and reaches a maximum near the right side of the curve. A motor will typically be rated at a service factor HP (SFHP) or total HP (THP) which is greater than the maximum BHP required for the pump. The SFHP/THP is the true measure of a motors power capability and is determined by multiplying the pump's service factor by the label HP. This is where a lot of the confusion originates. As a general rule of thumb, here is how a pump's service factor relate to the ratings:
Full Rated: Service Factor > 1.3
Up Rated: 1.0 < Service Factor < 1.3
SPL Rated: Service Factor < 1.0
Within the same pump line, sometimes you will see two pumps with different label HP and service factors but identical head curves and identical THP (service factor * label HP). Don't be fooled by this, if the head curves are the same, the pumps are identical. Only the labels are different. So the lesson here is that you cannot judge a pump's power or strength just by just the nameplate HP rating.