[Naru]

You increase your skill rating by using that particular skill. Each time you get 10 increases in your major skills, you gain a level, and can increase three of your attributes. But, you can only increase each attribute 1 + the number of skill increases in a skill influenced by that attribute, up to 5. So if all ten of your skill increases for a level are in blade, you can get +5 to strength, but only +1 for the other two attributes. Unless you are very careful about which skills you use as you level, you can't play on higher difficulties without getting your ass kicked because your attributes don't grow as quickly as they could, and you won't be able to get them very high before you run out of major skills to increase and can't level any more.

If you're slightly OCD like me, you end up grinding certain skills so that you don't accidentally level while doing fun things, but gain minimal benefit from it.