Symptom
After updating to Mac OS X 10.1.3, your display mirroring or extended desktop settings may have changed. Displays in extended desktop mode may revert to the default left-to-right layout. You may notice changes to display resolution, color depth, or refresh rate.
Solution
Mac OS X 10.1.3 includes changes to video device drivers that improve the reliability of computers using display mirroring. Due to these improvements, the system may not be able to retain the exact setting you had previously. You should restore your preferred settings using the Displays pane of the System Preferences application. Once you do this, your settings will be retained as expected.