WPILib Mecanum Function Not Strafing Properly

[CodeMonkeyMatt]

Hi,
We solved this problem a while ago with a code modification, but after reading some posts here, I want to find the problem. It seems that the WPILib RobotDrive mecanum functions work pretty well for most teams, but for some reason, when we tried it to strafe with it, two of the wheels turned in the wrong direction, thereby making the robot just jump and shake as all the wheels cancelled out. We got some code from chiefdelphi that takes the joysticks and uses that to calculate our mecanum drive, to the same effect. When we flipped the "strafe" joystick value on two of the wheels' inverse kinematic equations, it seemed to work. Just wondering why we would have need to do that.
I'm always happy to provide more information if I can, so just ask.