The aspect ratio defined in Project / Project options can vary from one project to another.
I often use 16/9, but also 16/10 so that I do not need to crop too much my photos which have a native aspect ratio of 4/3 = 16/12.
When export slides to Images, it would be nice if the current project ratio was used for the resulting image. Presently this is not updated when using this new (and most useful) feature. It can be manually input, for sure, but then this value "sticks" for the next project where it may not be correct.