Lack of convergence normal results as a product of the lighting setup. Convergence is achieved when the result of the latest iteration aligns with the current image. If you have competing light sources, the pixels in the iteration will often yield a different result dependent upon which light source it originates from.
Trying to achieve nearly 100% convergence on a render, for the most part is fairly pointless. Seeking to achieve this goal will force you to use very simple light configurations, which often detracts from the final artistic result. At the end of the day the final image is what matters and it doesn't really matter how you get there. Rendering in layers / down sampling / heavy post-processing are all fine in my books.
PS: The 3D Software Help thread is probably a better location for this query -
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