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Those are all sizes that are already generated for the used image in the WordPress Media Library AND have the same aspect ratio.

The File Size Optimizer looks at the required media size throughout all device layouts and in that way evaluates the available Dimensions.
If available, there is always a choice shown that is the closest to 1X or 2X.
If no fitting size is available, you can hover the standard (1X) or retina (2X) lines to get an info on the currently optimal size for your media.