In the leaks bar you see the orange spikes which show you the occurrence of the leak (the point in time it happened); the blue below it is an indication of the total leak amount which builds over time - you see it continue at one level for a while after the first leak, then you have the second leak and the total leaks display grows.
To get more detail double-click on the items in the list view. You will drill down to more levels of details and hopefully come to a view which will help you find the leak. But it's nowhere near as easy to see what is wrong as the little blue arrows that you get with a static analysis build...
The best approach I think is to make sure you are running Snow Leopard, the latest Xcode and start using static analysis. Since I started using it I found leaks in older code that Instruments hadn't revealed (probably my fault in not using it correctly, but anyway...) and I have found no new leaks in new code. The worst thing about it is false positives but it certainly kills leaks.