Though the Columbine Report was billed as being 11000 pages, the CD-ROM that Jefferson County sent out when it was released to the public originally numbered up to 10937. They later released several more pages which picked up at report page number 11139 and went up to page 25889, skipping over 12976 to 15911 entirely.
In 2006, more pages were released in lieu of releasing the Basement Tapes. This set of reports consisted of 936 scans and reports JeffCo called the "Columbine Documents", which begins at report page number 25923 and ends at page 26859. When they were released, people referred to them as the "1000 page Columbine Documents".
In combing through the various PDFs, I've noticed some other weird numbering discrepancies here and there. One example is that there are two sets of documents numbered JC-001-012238 through JC-001-012296 -- one set is the Team Reports and the other is a mix of witness interviews and transcripts from REB and VoDka's chat sessions and such. I don't know how JeffCo numbers their reports, so I can't explain what's behind the sequencing issues.