The numbers seem low, but I wouldn't be surprised if the XBL numbers are hugely inflated. Think about it - there's conclusive evidence that Team Fortress 2 has sold millions on PC, yet at its peak "only" ever saw probably 30,000 concurrent players. Left 4 Dead is also up there in sales numbers but with just as "low" concurrent numbers.
When you put it in perspective, though, 18,000 is still a huge number. Again, that's just concurrent - not total unique players per day. When you start thinking of average play session, total number of hours per day, how many tens/hundreds of thousands of unique players per day that a peak concurrent number can represent - gets pretty huge.
I suppose. I guess when I see Quake Live and W:ET (my main game for many, many months in the case of latter) getting ~10k users peak, I expect a lot bigger player count out of a retail game. Obviously, being free will change things up, but America's Army is/was free and that didn't seem to have as big of an impact.
And yea, XBL inflating their numbers is possible. I don't remember my source, and they don't exactly have a public server people can ping for player count or anything.