Only Autos get 21 MPG, and even then only if they are driven gently. The 6 speed manual will never get better then 18. My 6 speed was averaging around 15, but as the engine wares in more, and I stop putting my foot to the floor every chance I get, It's now getting closer to 16-17. You'll only get 18 in a 6 speed manual if you drive on the freeway all the time at no more then 60 mph.
That's my experience anyway.
P.S. You need to make sure you're calculating your fuel correctly, and averaging over time, not just going off of one tank full, to truly get accurate MPG calculations. Personally, I use a program on my Treo 700p (palm OS) to accurately track milage and expenses related to my vehicles, but you could do it with a simple spreadsheet or calculator as well.