mac memory does not add up - exactly 1Gb missing

I just upgraded to 4 gb of ram on my Macbook pro, and in the system profiler, 'about this mac', and activity monitor all say that yes, I have 4 gb. here's the strange thing though.

TOP says,

146M wired, 336M active, 22M inactive, 1540M used, 2556M free.

BUT, the wired + active + inactive to not sum to the used as they should. Furthermore, activity monitor displays the same statistics, except

146M wired, 336M active, 22M inactive, 504 used, 2556M free.

indicating that it just sums the wired, active and inactive, again leaving 1Gb unaccounted for. Where is my other Gb? Top says it's used, but, where? and the activity monitor just has no clue!

Any help would be super appreciated! Thanks guys!

Which model Macbook Pro is it? Screen Size? CPU Speed and C2D? Early Core 2 Dual MBP would only really use 3GB RAM even if 4GB were present. How old is this MBP?

oh gosh, it's a 2.16GHz Intel Core 2 Duo. so yeah, it's an earlier core 2 duo. the MBP is 2 years old or so, so that makes sense. How strange. Any idea if having the 2 2Gb side by side is helpful at all? I've heard having identical memory sticks is more efficient... Is ahving the 4 Gbs going to be detrimental at all perhaps?

From what I have heard it will work fine but you will not get the full 4GB as you have seen. Unless someone else that has tried it says different, it should be fine. I like having Matched Pairs of RAM in my systems personally if the system supports it.