Weapons, armor and jewelry also add to your stats. Your total strength will be the sum of points you have added plus any points your equipment adds thru magic etc. If you have a pair of boots that add several points to strength . . . and swap them out for a pair that has more armor protection . . . your strength will drop and your defense will rise and any equipment you are wearing that depended on a certain strength stat level will no longer be usable.
If you equip, or in some cases, have a cursed item in your possession, your stats will drop. One cursed axe in particular will drain your stamina continuously just having it in your inventory.
I have found certain items that enhanced my stats greatly and wore them for many levels even though I had found a "better" item, removing the item caused my stats to plummet. Look closely at the items you have unequipped or equipped as they affect other stats. Unlike other RPG's , DD will give you some excellent items that can / will be better than anything you find for many levels.
I use one sword for almost the entire last half of the game because I have found no better weapon. I like this because it allows me to focus on the story rather than constantly looking for better equipment. I do look at everything I find but it must be an overall benefit rather than an immediate benefit to one stat while lowering other stats. Of course, if the item increases a stat or protection but lowers no other stats . . use it. The Dragon armor breast plate is another example of a item that adds lots of points but offers little damage protection compared to other breast plates you are finding. It's another dimension of the game . . . =)
Edit: open your stat window when equipping items and click on the new. This will equip the item and change the stats. Clicking on the unequipped item will re-equip it and allow you to see the stats for the old item. Clicking back and forth on the old and new items will allow you to easily see the benefits of the new item compared to the old item. Most of the time this is not necessary since you will be able to determine if the item is a benefit or not just by looking at the item description.