I've heard this a handful of times too, from family, friends, and even one medical professional (an optometrist). Many times I've found myself saying, I'm making progress, but I still have a ways to go before I'm 100% (this is more or less my mantra).

I think people say it for a number of reasons - for example, to make you feel better; to reassure themselves; to attempt to create a sort of atmosphere of hopefulness; ignorance about the nature of brain injuries; or the perceived need to say something to avoid feeling awkward or uncomfortable.