why is a study submitting a single end file for each pair end file?

[computational_m]

Hello,

I was checking out the sequencing files for a microbiome study on NCBI. There is something I do not understand / I can't seem to find anything about it in the publication, that is they have a single-end reads file for each pair-end reads file even though the publication only mentions having used pair end.

I am confused as to why did they do that?

Thank you