Kumārila's obscurity and Vācaspati on it

Why did Kumārila choose not to explain what he meant with “artha” in arthabhāvanā or which kind of compound is śabdabhāvanā? And why did he offer two explanations which are even more obscure, such as arthātmikā bhāvanā and śabdātmikā bhāvanā? The couple has been repeated in mantrādhikaraṇa and bhavārthādhikaraṇa (at least), hence it is not a badly deviced hapax legomenon.

In a passage pointed out by Harikai (1990:209), Vācaspatimiśra comments Maṇḍana's proposal of better discriminating (pravivic-) the prescription and rhetorically asks what is left to discriminate after Kumārila's and his followers' analyses. His answer is:

kṛto 'pi tair viveko 'kṛakalpa eva | “abhidhābhāvanām āhur” ityādisaṅkīrṇaśabdaprayogāt

(Nyāyakaṇikā ad Maṇḍana Miśra's Vidhiviveka, 1907 Banares p. 4, l. 17)

“Although they have done the discrimination, it is almost undone, because of the usage of confused expressions such as “[optative and other endings] express the designation-bhāvanā …”

Obviously enough, Vācaspati is not entirely reliable, since he wants to interpret Kumārila as maintaining the same view proposed by Maṇḍana (according to which the prescription conveys the idea that the action it enjoins is the means to accomplish something desired).