XPM & Smokey have it right, but WAIT! There's MORE!
Early engines had (mostly) a linear throttle-to-rpm control characteristic - it's simpler mechanically.
But by the late 50's it was clear that this increased the pilot's mental workload a bit, and the systems guys decided the "speeder spring cam" (that links the throttle handle to the governor speeder spring) needn't be linear - it could just as well be shaped to give a large rpm change rate near idle, and a more gentle change near the top end. In so doing, the throttle-to-thrust curve can be made very nearly linear. Pilots like linear response! I think most engines built in the last 40 years have the "smart cam" or its electronic equivalent.
There's another aspect to this - the dynamic response of the engine. Early engines were notoriously slow to accelerate. If you were low and slow on approach and gave it some throttle, it seemed like forever before thrust started coming up. That made the early throttle-to-thrust characteristic SEEM even worse! Again, modern engines have a much friendlier response rate so it's no longer a big issue.