Those are all sizes that are already generated for the used image in the WordPress Media Library AND have the same aspect ratio.
Why are my choices not exactly 1X or 2X?
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.
Please check the listed license files for details about how you can use the "FontAwesome" and "Stroke 7 Icon" font sets for commercial projects, open source projects, or really just about whatever you want.
Please respect all other icon fonts licenses for fonts not included directly into Slider Revolution.
Further License Information
Font Awesome 4.6.3 by @davegandy - http://fontawesome.io - @fontawesome License - http://fontawesome.io/license(Font: SIL OFL 1.1, CSS: MIT License)