rebase -i reword runs pre-commit hook with curious results

I'm confused on why and/or how interactive rebase runs the pre-commit hook when doing the reword command for commit (a). My pre-commit hook does keyword expansion on the worktree copy of the modified index files and then re-adds them to effect a user-date-stamp when committed. However, the user-date-stamps don't get updated when reword runs the pre-commit hook. IOW, the pre-commit hook does not get the same results as if I were doing a commandline git-commit of a modified index. I suppose reword is protecting the preservation of all the contents of commit (a) except the commit message which makes sense, but I don't understand how it goes about doing this while still attempting to somehow honor the pre-commit hook. (git 1.7.1)

v/r, neal