That is actually pretty normal. Most 2-stroke gas engines are at full power before the throttle plate is fully open

There are several ways to get the stick to be a full rpm at full stick.

1) Use a throttle curve if you TX supports its. Some do and some don't. If not then use a programmable mix that supports a multi-point curve, then do a throttle to throttle mix and do a curve there. Find out what max RPM you are getting while at full stick, now while in the mix, slowly raise/lower the curve (depends on the TX brand/type) until you hear the engine start to drop rpm then bring it back up to full rpm, Now move your stick until you get 1/2 the rpm you had at full stick and move the curve to make the stick be at half way, then move the stick to idle and set the curve to get the idle you want. Now if you have peaks and valleys between those points, then using the multipoint curve clean them up so it is a nice line from low to high. Now your stick with show an rpm increase all the way up.

2) You can also get very close mechanically, as an example, If your carb arm is one inch from the point when the control arm (clevis or ball link) connects to where the rod is in the carb for the throttle plate (pivot point), then you want a servo arm connection that is roughly 3/4ths of an inch from the servo arm screw (pivot point) to the control rod connection (clevis or ball link).

I personally use the throttle curve method. As the engine breaks in it's easy to reset the curve to get the rpm increase you will see and if you go to a bigger prop you will need to retne and set the curve. It's as easy as changing the TX versus possibly having to chage the servo arm connection. Plus if you do some hovering you can flatten the curve around that point to make the throttle less sensitive.