200LRs and 200Fs have a "Vref Min" feature. Being the short fuselage with the large engines, Boeing feels they need a minimum speed on approach to ensure control authority to handle the asymmetric thrust in the event of a single-engine go-around, as the rudder is acting through a shorter arm than on the 300ER
As thrust decreases with pressure altitude, the minimum approach speed like-wise decreases as PA increases.
The minimum Vref is approximately 136.4Kts IAS at sea level. Below a certain landing weight (about 195T) Vref 25 will be the same as Vref30 at sea-level, so it is good practice to land flap 25 so you are nearer the "actual" vref- increased landing distance due to less drag is minimal and offset by the decreased likelihood of float.
The 300ER doesn't have this feature as it has sufficient rudder authority at the normal Vref at its' a lightest weight, due to having the longer fuselage and thus longer arm.