Oh, I found this - is this a correct explanation? "The t-less form of such adverbs is obligatory when the adverb is isolated (i.e. with no corresponding adjective) or the meaning of the adverb is essentially different from that of the adjective (e.g. endelig, "finally, at last" ~ endeligt, "definitively"). In other cases, the t-less form is preferred when the adverb qualifies an adjective (e.g. væsentlig(t) større, considerably larger")."