Is there anything 'strange' about the 'Original' 13 files that are missing the export?
Are these 13 originals marked as "Missing" with an [ ! ]?
Are they perhaps movie/video files?
Do you get any error messages?
Have you tried exporting from the 343 again to a different folder?
What are your Export settings? File-type, etc?
The 'Sort Order' of files is entirely dependent on the App or software that you use to view the files. LrC may be sorting by 'Filename', the App you view the exports may be using 'Capture Time". So how do you expect the files to sort?