Why the discrepancy between horrific and horrible and terrific and terrible?
Do you refer to two separate pairs of words that have different meanings, or to four words that have, at least in one sense of usage, common meanings?
Ask the Romans. They’re the responsible ones:
horrific - from the Latin horrificare
horrible - from the Latin horribilis
terrific - from the Latin terrificus
terrible - from the Latin terribilis
American Heritage Dictionary
Presumably the OP means why do “horrific” and “horrible” mean much the same, where “terrific” and “terrible” are opposites?
In ther past (e.g. OED 1964) “terrific” did mean “causing terror”. I’d put it down to a common usage drift where some words with a negative meaning acquire, initially as slang, a positive one: e.g. “terrific”, and more recently, “wicked” and “bad”.
The OP isn’t the only case where symantic drift has created counterintuitive antonyms.
Consider the situation of awe and wonder, awful and wonderful. The first two are synonyms and the second two used to mean “inspiring awe or wonder”. Drifting meanings resulted in the latter now being antonyms.
Because English is a living language and not some sterile machine-design.