Here's what I think it
should mean. Most (I never say all,

) children level out
to their ability level by 3rd grade. And by this I mean that bright kids that start slow are probably going to be on track by about 3rd grade, even if they putzed a bit getting started. And some of those kids might even test gifted, despite a "late" start. And other kids who were hot-housed into reading half-decently by 5, are probably going to be just as good in 3rd, as they would have been without that extra effort.
This has been our experience too. The kids I
know are bright but just aren't showing it in the usual ways early on, start showing it by about 3rd grade. And the kids whose parents pushed and shoved every scrap of "learning" they could down their throats early on aren't seeing the same leaps ahead that they might have when they were preschoolers. Also by 3rd grade most kids
are reading, so the difference between reading fluently and with excellent comprehension at an adult level vs. just barely reading isn't quite as noticeable as the difference between just reading vs. not.
With DS, what I've found is that by 3rd grade he was much more "even" with himself. The areas that he had lagged behind suddenly started to catch up (coughcoughspellingcough), and the unpredictable pace at which he had been progressing started to smooth out into about a year's growth in a year's time. Just not the particular year one would expect of an 8 year old.

So part of that could have been my settling into the correct level of instruction in finding just what amount of challenge it would take him a year to conquer, but I think there was something developmental in there too. Just not the same "evening out" that people are usually talking about when they say that!! LOL