The quick answer is that while the Greek says “kecharitomene,” the Vulgate translates it “gratia plena,” which is “full of grace” in English. This phrase is not uniquely applied to Mary, but also to the Word (John 1:14) and Stephen (Acts 6:8) and maybe in some other place I do not know off the top of my head. “Highly favored one” probably corresponds closer to the Greek grammatical form, but it sounds awkward and was probably formulated diliberately to undermine Catholic Marian doctrine. I’ll let others more knowledgable expound the meanings and their theological implications.