Experience from observing simulator sessions in the 737 in Australia, reveals that many pilots do not know the exact position of the reverse idle detent. Because of this they will often move the reverse thrust levers fully down, instead of stopping their movement at the reverse idle detent. When this happens, the engines are still spooling down from high power - with the result that when out of reverse, the engines are still delivering significant forward thrust while in the process of spooling down to idle. This causes momentary acceleration of the aircraft which increases the landing roll.
See it all the time, on the line as well. In some cases it is not that they don't know where the reverse idle detent is, it is just that they are not paying attention to the engine indications as they come out of reverse & haven't ensured that they have
idle reverse before selecting forward thrust.