The Septuagint is not one simple, uniform ancient manuscript. It refers to Greek Jewish scriptural translation and textual traditions that developed over time. Brenton brought a printed Greek edition into English for nineteenth-century readers, and his work became a familiar English Septuagint in traditions that use Greek-scripture books.
A difference between Brenton and a Hebrew-based English Bible can arise from the Greek translator’s rendering, a different underlying Hebrew reading, or later textual transmission. The comparison is informative but does not by itself prove which witness is earlier or more authoritative.