It's because you have numbers above 255 being packed as "C", which is supposed to be for bytes only. Perhaps using "U" will do better?
Update: see, this is why I should click "Submit" right away, instead of being distracted.
Jeff japhy Pinyan,
P.L., P.M., P.O.D, X.S.:
Perl,
regex,
and perlhacker How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart