Ok, I was hoping someone else would comment on this, but here's my guess:
First, I will assume that top right corner is T19. Then, black can pretty much force this sequence if it's their move:
(Black) T12 T11 T14 T15 S17 S16 T17
After which white cannot answer T16 due to T14 eating white group, and cannot play T18 due to T19, thus two black stones on S17 and T17 live, and white side dies after black add T18 and T19 (bottom half has just 1 eye).
So, I guess evaluation software sees this killing sequence for black if it's blacks move, and thus knows white needs one more stone to truly live there. Hence thinks that territory is 'undecided' yet.
Let me know your thoughts.