Scratch part of that. The part of the datasheet you posted says it will output a complete word, so those extra 1's are coming from that. However, the last two F's would mean that all datalines were active. That's unlikey. Even if your address was 15 (F) meaning all DIP switches of the address were set, here is what it should look like when you press a button:
(the first FF part would be your address part)
FF01 = 1
FF02 = 2
FF04 = 4
FF08 = 8
FF10 = 16
FF20 = 32
FF40 = 64
FF80 = 128