Those buttons are merely electronic signals that tell the circuitry to send less (more) voltage to darken (brighten) the backlight. Sure, the button presses are obviously intercepted by the OS first, so there is a possibility of a driver or OS issue, but considering it is likely no one else in the world has this issue (google search), that possibility is next to zero. There aren't many versions of MacBooks (hardware wise) and Mavericks easily supports all the ones made during/after 2008, and Macs have extremely standardized hardware (and thus standardized software and drivers).