Nikon D90, when using continues mode, why each photo exposure is different?

I have Nikon D90, and when I'm using the continues mode and taking photos continuesly with the same aperture and shutter speed, each photos with the same lighting has a different exposure. Why is that? And how can i avoid this without holdin AE-L?

Without seeing sample shots this will be difficult to tell.

1. Are you sure that they are using the same shutter speed and aperture, i.e. Are you using full manual mode?
2.Is bracketing deactivated?
3.is the ISO automatic deactivated? It may be that it's messing around with you. That you have to hold the AE-L button indicates so. Are you by any chance using spot metering and/or moving the camera while you're shooting continuously? It may be that you're getting different meter readings for each shot that way (especially with spot metering), and the ISO auto is working in the background trying to give you a proper exposure each time. Due to different meter readings that the camera works with that then may result in different exposures each time.
Check if the photos were taken with different ISO values. If the values differ, then that's the culprit.

Are you using the flash? It can't recycle as fast as your frame rate so you would have some photos with flash and some without.

Put the camera in manual mode and it will work properly. If you use anything else, the camera meters each individual shot, and depending on what it sees, the exposure will change. The only way to prevent that change is to do things yourself, without the camera making decisions for you. Shooting in green box auto or P mode is the worst way to do any sort of continuous shooting.

Jens has a good answer. I suspect that you have a bracket set without realizing it. Push and hold the shutter release button in for a few seconds. Does your camera pop off the same number of frames each time? It would take a fixed number like 2 or 3 or something and stop if you have a bracket series set. Turn off bracketing and you should be good to go.

Push the BKT button in and look at the top LCD display. If it says anything other than "0" you have to turn the command dial to make it say "0."