Why is navigable correct?

I don't have an answer for why navigate yields navigable instead of navigatable; but in doing so, navigable is by no means a rule breaker among its closest peers. In fact, English has at least eight word pairs involving verbs with a -gate ending and adjectives with a -gable ending, as confirmed by The American Heritage Dictionary of the English Language, fifth edition (2010):

• investigate --> investigable

• irrigate --> irrigable

• litigate --> litigable

• mitigate --> mitigable

• navigate --> navigable

• obligate --> obligable

• propagate --> propagable

• segregate --> segregable

AHDEL doesn't list any similar -gable adjective form for a larger group of at least seventeen -gate verbs:

• abrogate

• aggregate

• arrogate

• castigate

• conjugate

• congregate

• corrugate

• delegate

• derogate

• elongate

• fumigate

• instigate

• interrogate

• relegate

• subjugate

• subrogate

• supererogate

But on the other hand, it doesn't list any adjectives of the form -gatable. For these reasons, it seems fair to say that, within the subset of -able adjectives associated with verbs ending in -gate, navigable follows the standard pattern—the same one that the other seven -able adjectives associated with -gate verbs follow.