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My original answer below is wrong, see the correct answer of Rob Jeffries. The author of the OP 0x90 asked me to not delete this answer and instead explain why it was wrong. Perhaps this can be a teachable moment in how the EHT pictures can be misinterpreted.

This is because of the tilt of the accretion disk with respect of our line of sight. The accretion disk is "in front" of the black hole in the southern part of the image.

You can compare with this image from the 5th paper of the ApJ Letters "First M87 Event Horizon Telescope Results. V. Physical Origin of the Asymmetric Ring", which shows one of the best-fit theoretical simulations alongside the observed image:

There is a number of other effects at play, though. You see only a narrow frequency band, so once the light is shifted by the Doppler effect to a different wavelength in either direction, or when the plasma has the wrong temperature, you stop seeing it in the image. An additional general-relativistic effect is a "space-time push" the rotation of the black hole gives to co-rotating photons, which causes a slight west-east asymmetry in the image as well.

Explanation why the above is wrong:

As already noted, the what we see in the EHT image reconstruction is not just any image, it is a representation of the strength of a radio signal at almost exactly 1.3 mm in wavelength. Even though it is rendered in yellow to orange color for intuition, it is not an ordinary image of an object over a continuous spectrum and the radiation of the plasma ends up being much more directional and specific than you might usually expect. Another ingredient that greatly complicates the understanding of the image are relativistic effects. For example, often the emission from behind a black hole turns out to outshine anything in front of it! Even judging a slightly blurred numerical simulations without additional knowledge requires a great deal of direct experience with the problem.

The crucial mistake I made was the assumption that the image would always be rotated so that the assumed spin axis is pointing north. This is how every numerical simulation result is presented and I have been fooled by doing too much theory to think differently. Naturally, if they do not even know where the spin axis is pointing and have at best a blurry idea, it is better to plot with respect to a standard system of celestial coordinates, which happened in the EHT image.

Now let me expand on Rob Jeffries answer to contribute to the picture of how the image emerges. I extracted and rotated this picture from simulations of Monika Mościbodzka and other EHT collaborators (2014, A&A): In this picture you see simulation results where the jet as well as the black hole spin are tilted in various ways with respect to the observer (the jet and spin are always aligned here). The tilt is the same in every column, and every row represents a different magnification and wavelength of observations. You see that when the plasma is moving toward you, it appears brighter. This is relativistic Doppler beaming. There are various components to the image, including not only the disk but also the jet.

If the disk was edge on, you would see a ring created by the jet (plus disk-to-jet transitioning matter) that is stricken through by the main disk. This corresponds to the right-most column. As you rotate more and more (but not quite) face-on towards the disk, there will still be a component of the plasma that ends up being strongly beamed towards you while the whole image starts looking quite a lot like a pure ring (left-most column). This is the current leading scenario for what EHT saw in M87.