You need 5 shield skill for the huscarl's round shield. All difficulties requirements are determined independently from each other, so you do not sum them like you posted. That is to say, having 15 strength would let you use any combination of armor pieces that have a difficulty of 15 or less, but you could not uses any armor piece with a difficulty of 16 or more.
Difficulty for horses is the amount of the riding skill needed.
Difficulty for body armor, helmets, gloves, boots, one handed weapons, two handed weapon, polearms, and crossbows is the amount of the strength attribute needed.
Difficulty for shields is the amount of the shield skill you needed.
Difficulty for thrown weapons is the amount of the power throw skill needed.
Difficulty for bows is the amount of the power draw skill needed.