I think the answer is simple. Monier's dictionary is basically a translation of Boehtlingk's PW, as Dr Dimitrov has pointed out only recently. One of the consequences thereof is that Monier has also adopted Boehtlingk's order of meanings, which generally depict the chronological sequence and related textual categories of his sources. In the present case, evidence for the first meaning of aṭṭa ("watch-tower") was found by him in the Rāmāyaṇa, the second in Hemacandra's Abhidhānacintāmaṇi. A close perusal of Boehtlingk's prefaces to his own dictionaries (PW & pw) and the principles he followed will possibly clarify also the matter as far as Monier-Williams' plagiarism is concerned. He might however have left an information about the way he structured the order of meanings in his preface.
Kind regards,
WS
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Prof. Dr. Walter Slaje
Hermann-Löns-Str. 1
D-99425 Weimar
Deutschland