Throughout history, knights, soldiers and warriors have learned a variety of skills. A knight could be a master swordsman but had also trained in hafted weapons or Ranged combat. This cumulative experience is called Prowess. As you progress in Melee or Ranged or Magic, your Prowess will increase, showing your overall effectiveness with all skills.
Melee: Just because you have never used a flanged mace before, doesn't mean you haven't encountered it before in combat or maybe you have used an axe before and they operate them same in regards to point of balance and functionality. Instead of penalizing you when you use a weapon you have no Ranks in, instead you are given a small bonus of Ranks from your Prowess to show that you wouldn't be a novice with it.
Ranged: While it is true that it took years to learn to shoot a bow effectively and little to no experience to shoot a crossbow effectively, the overall understanding of flight times, gravity, distance, and target movement, is universal and moving between the two Launcher types would translate with each other.
Magic:
Prowess is a bonus added to all your skills, even ones you don’t have points in, in the form of Ranks.
Every 10 Ranks you purchase from Melee, Ranged, Magic, Secondary Skills and Styles, adds 1 to your total Prowess.
Prowess follows the same rules as purchasing Ranks, that you get Expertise choices at every even Rank purchased. Due to Prowess being a global mechanic, you are given Expertise choices that are Prowess specific.