One of the European Blizzard forum mods posted an excellent explanation of how +x to ability items work (Example: +10 to healing spells). The beginning of the article is quoted here:

When an item has the effect "Increases damage and healing done by magical spells and effects by up to 20", the damage dealt or hit points healed by one of your spells will be increased by a fixed amount up to 20. The actual amount that the spell is increased by is not determined randomly, but by which spell you are casting. You are not receiving a random bonus value between 1 and 20 whenever you cast a spell. Some random items, such as those with the property "of Shadow Wrath", indicate that they provide "+X Shadow Spell Damage". The actual amount of the bonus is again determined by the spell being cast. There are a few items that do provide a random bonus, such as Dreamweave Gloves, but their effect notes the randomness by stating that item's bonus as "up to 1 to 19." Once again, the bonus received could be reduced depending on the spell being cast.

The complete article can be found here