There is a range of reasons for it, and only one of them is a bug.
First of all, growth rate has nothing to do with how much you deliver on our normal CB servers. You grow or you don't, that's what meeting the requirements does.
Growth rate is (mostly?) influenced by the amount of houses and how much the town likes your company.
It seems to me the town likes you though, and 23 houses should definetely give you a higher growth rate. The best way to tell is usually if the population grows, however slow, but the requirements remain the same. 17 mail seems too low to me, I'll come check it out in a minute.
If this happens, call an admin, even when in doubt. I can imagine one might not have been online, so thanks for posting this!
Edit: It should be fixed now, thanks again, for reporting the issue