Totally Normal, and Real2You posted a very good first response to your question. Adjusting the throttle curve is your first step. that is assuming that your pitch curve is close to where it needs to be.

Now tell me, how did you measure the rotor speed? Did you do it by yourself? Did you have somebody help you? and most scary of all questions: did you have the helicopter tied down to a fixed object? It is pretty hard to measure the headspeed of a climbing helicopter (when pitch is added helicopters climb). We normally measure headspeed of a hovering helicopter and "gage" the performance in the climb and adjust from there.

Rafael