Because that isn't the word they are trying to teach.
In practise it should accept it. Next time put tofauti and, when marked wrong, report "my answer should be accepted".
What Juryrigging said is perfect. What I also wanted to point out is that mbalimbali is like "various", meaning many different various things. I.E. you can't say "That's different" using mbalimbali, that would have to be "tofauti" ...
... makes no difference to your question since the English word "different" without any context doesn't indicate which one.
That's my biggest gripe with this course: so many "sentences" are wasted on single words or contextless fragments ... it's more or less like doing flashcards. Basically all of the other courses use mostly full sentences once you get past the absolute basics.