I had a look again at Delta-1 chess, I know now what it is:

They (some Scisys engineers) took the rom of Boris Master (rev. 1) and tried to rewrite it for a 4x 7-Segment LED, unfortunately they botched the hack job,
the timer interrupt calls the new display routine much more frequently and so looses time for chess calculations; and more weird stuff is going on.

No wonder it plays so weak.

In the Delta-1 ROM is even some fragmented code remaining of the message:

0878:BORIS AWAITS YOUR MOVE
01 CD 7E 53 09 37 AE 50 (BC FD 59 C0 86 0)
(snipped from Sean Riddle's disassembly of Boris rev. 1)