A) no 2 monitors display exactly the same and there for the picture wont match 100%?
B) the dimmer monitor is defective and there for not as bright as it should be?
C) windows isnt calibrating them properly or maybe a color profile issue?
A.) No two monitors are the same. Period. If you get monitors from the same manufacturer or more specifically the same model, you'll have a better chance of them being close. But there are always manufacturing tolerances.
B.) Possibly. Could your other monitor be defective and be brighter than it should be? Possibly. You can try turning down the brightness in the monitor's OSD if you haven't done so already.
C.) Windows will treat the monitors the same, it doesn't "calibrate" anything for you. Check in your graphics card's control menu to see if it has tweaked the brightness to be different (although if the backlight is brighter the computer doesn't influence that at all). It's not a color profile issue, as color profiles only deal with displaying color differences through color managed programs (not luminance).
Double check your GFX vendor-specific control panel and mess around in the monitor's OSD. Otherwise you can try to return it for another one.