Fulcrum paddles let you push harder, safely. That may not sound very significant but it is. The Fulcrum paddle is designed so it can be moved very easily from the entry position down to the catch position. The Fulcrum paddle is also symmetrical. Asymmetrical paddles cause twisting in the shoulder joint and may eventually lead to injury.
The weight ranges are rough guidelines. If you are an excellent swimmer and have healthy shoulders, you will be able to use a larger size. If you are an average swimmer, then it is best to start with a little smaller paddle. The important goal is to avoid injury.