Yep, it’s us hidebound lawyers at work again.
The use of parenthetical numbers is an historical holdover from the days when legal documents were written out by hand. Of course , not everyone has the greatest handwriting in the world, so it became customary to both write out the number, and to use arabic numerals, to lessen the chance that a particular number might be read incorrectly. (This was particularly important in deeds, where the measure of one’s real estate holdings were set out with verbal descriptions. Reading the footage incorrectly could cause some real problems–overlapping property lines, angry neighbors, misplaced spite fences, etc.)
You can also see the phenomenon on bank checks. When you write out a check, you give the amount both in arabic numerals and in “written out” form, to lessen the chance of an error in the amount paid.
In printed documents, I believe it’s fair to say that the continued use of parenthetical arabic numerals is an anachronism.