Light Intensity vs Visibility/Brightness in Double/Single Slit Patterns?

If you have ever tried to take a photograph of an interference pattern you will soon become aware of the difficulty of obtaining a picture of the pattern as "good" as that which you can see with your eyes.

You will find that if you get the exposure of the central part of the pattern right you cannot see higher order fringes.
Adjust things so that you get nice higher order fringes and the central part of the pattern becomes overexposed.

Then you can have a similar problem when displaying the image on a computer screen.

Here are two images which illustrate the problem.

The top picture shows the Sun correctly exposed but the building under exposed whereas the bottom picture shows the building correctly exposed whereas the Sun and the surrounding sky is overexposed.

The dynamic range (ability to process a range of light intensities) of a camera and a computer screen are far inferior to that of the eye.
So your picture of an interference pattern is not a true reproduction of the actual real life intensities.
The chances are that it was either computer generated or processed to show the salient features at the expense of getting the intensities wrong.

High dynamic range imaging has been made easier with the advent of digital cameras and computers where techniques are used to improve the quality of the final picture.

An example from the Wikipedia article shows how a number of images can be combined to produce the "desired" image although you must remember that the image you see might be degraded by the poor dynamic range of your computer screen.

Without any image processing you might get something like this:

.

So now take another picture with the central fringes correctly exposed and combine the centre of this image with the outer parts of the image above and you have a nice representation of the interference pattern that you can see with your eyes.