Growth rates change from time to time.
IMO....the practice of 'power feeding' to speed up the process is wrong and eventually leads to the grossly overweight specimins that look like footballs with fins that you see far too often. Lucy (the RTC in my avitar) has this same...On and off growth rate thing going on...she'll seem to stay the same for a while and then gain an inch or so overnight (looks that way in any case...LOL)
Houdini...My MRTC (Marbled redtailed catfish) is another story all together. he came home at about 4 inches and a month later he's just shy of 7 inches.