By an answer provided on the imslp forums, it seems to be an internal numbering, without any particular order or meaning:
b) IMSLP is the authority on I-Catalog numbers, but any composer is welcome to create his/her own catalog system (e.g. opus numbers).
c) To keep things simple the only promise is that the numbers are unique and stick with the page. The numbering is not intended to be chronological, rather, the initial assignment is ordered alphabetically. Sometimes this becomes chronological (e.g. Symphony No.1 through No.30 or something), but that's not the intention. Newly added pieces are assigned the next available number.
d) The intention of the I-Catalog system is as a fallback to uniquely identify pieces where there is no authoritative catalog system. It is not intended to be musicologically sophisticated or to replace a well-researched catalog.