what's wrong with my magic wand?

Whenever I use the magic wand (it seems) I always have the same issue. I start off being amazed at how magically it works (seriously) then I always get to a point on my photo where it starts selecting it how I want, then jumps the the edge of the photo and over selects by a ridiculous amount. I've tried tweaking several adjustments and approaches, but have the same issue. I'm sure I'm missing something basic here. I don't know if it's an issue with the photos, as there are pretty clear lines for the tool to go by.

I often have the same problem and seldom use it anymore, opting for one of the other selection tools that have more control. This gets tricky with my shaky hands though.
The magic wand seems to select by exposure or brightness.

Have you adjusted the Tolerance setting? The magic wand tool is often referred to as the 'tragic' wand too. It's a pretty basic selection tool that doesn't offer you a lot of control. If it's not working and you can't adjust it to make it work the way you want it to, you may have to find another way. Depending on your version of PS you could try the Quick Selection tool. If I have something with a nice clean edge on it, I will often use the pen tool and manually outline what ever it is I want to select.