Kurzweil keyboards have different tuning systems selectable, especially the K2000 series (they have about 18), and sometimes a different tuning system can accidentally be selected, making a difference across the keyboard. Equal-temperment is what is commonly used, with other forms of tuning having inherent problems with playing in different keys and octaves (which is why equal is used now). Simply select a different tuning system or try performing a hard reset of your keyboard.

For example, on the K2000 series (including the 2500, 2600, and 2660), you would go to Master and change intonation there (default is #1 Equal).