Because UA is a probability distribution which sums up to 1 (or 100%) purely based on account relationships, it's a zero-sum game: if one account increases its UA, another one will have it's UA scored decreased slightly. Following accounts is entirely democratic and dynamic rather than static: every follow and unfollow broadcasted on the blockchain (currently ~ 150 million) changes UA scores for all accounts.
UA doesn't have to dig into content itself. UA looks at the influence / reputation of the author and voters of content, instead of trying to analyze the content itself.