While not explicitly mentioned in the CSL specification, the citation-numbers of bibliographic entries should always be in order, even when the entries are alphabetically sorted by author (as is the case for the SIGCHI CSL style), so this probably should be fixed in the CSL processor.
[edit: this bug has been reported and has been fixed: https://code.google.com/p/citeproc-hs/issues/detail?id=83&can=1 ]