Rather than explain a metering phenomenom numerous times I will explain it here.
When using auto exposure (program mode, shutter priority, aperture priority), the metering system will always yield an image that is equivalent to a sheet of 18% gray paper. Most of the time, this works fine. However, when shooting bright or all white scenes or very dark or black scenes, the meter will yield the wrong exposure. In the case of a bright or white scene, the meter will darken the exposure to 18% gray. In a dark scene, the meter will lighten the exposure to 18% gray.
To overcome this problem with the metering, we use exposure compensation. Exposure compensation allows you to increase or decrease the metered exposure by a specified amount. In the case of a bright scene, it is necessary to use positive compensation. Start with a compensation of +1.0 and adjust up or down depending on the resulting exposure.
With a dark scene, you would use a negative compensation. Start with -1.0 and adjust as required.
Basically what this all means is that if you are shooting a bright scene, you need to open the lens an f-stop or more and for a dark scene, you need to close the aperture an f-stop or more. It sounds odd but it is true. Remember, this technique is only needed for automatic exposure.