[madnak]

The problem here is the adverb "not." It can modify either the verb "are" or the adjective "equal." Logically the former case gets you "the digits not (are equal)," the latter gets you "the digits are (not equal)." This is like the "spinning dancer" problem, people interpret it differently.

In the 990 case "not" and "equal" both modify "are." When "not" modifies the main verb, we can usually apply it to the whole sentence. The clause in this case. Applying "not" to the clause yields "it's not the case that the digits are equal." In the 770 case it's easy to avoid the adverb - substitute "not equal" for "unequal" and we get "the digits are unequal."

("All three" is redundant - I leave it out because the writer should have done the same.)

These situations are one reason why good writers avoid adverbs whenever possible. It also helps explain some stylistic conventions - "cannot" is clearer than "can not" because it prevents this sort of ambiguity.