This link talks about the synthesizer keyboard, here is a quote:
The synthesizer keyboard, on the other hand, is a diode-filled 49-key matrix. This allows you to press whatever keys you like, without restrictions, and without ghost paths. The piano keys are mapped to matrix points in a very straight forward manner, so I won't bother with a matrix diagram. The lowest note on the keyboard corresponds to Col 0, Row 0, and the highest note is in Col 0, Row 6.
At the start of the page, under Hardware background, it describes Introduction to Matrix Scanning which may be of some help to you.
Your problem, I feel, is possibly either a short circuit (s/c) key contact or a s/c between two key wires which is simulating it being on.
Hopefully this is of some help.