You caught us!
We do tend to slightly understate winds in order not to alarm the nervous passengers among you. And we are usually told not to use knots as many people aren't familiar with that unit. Here in Holland general weather reports/forecasts still only give the Beaufort scale(!).
Also I must say that quite often we are also surprised with how windy it is when we do the walkaround check - the wind report we get is often averaged out over 2 minutes at many airports and though gusts are also reported it does often seem windier than for eg 10 or 15 kts might suggest.
Plus the wind is reported from near the tower or met station and the terrain on approach may cause venturi effects between nearby hills or buildings giving a bumpier ride in the last few minutes compared to an open space.