I think I vaguely recall the explanation from a classics course many, many, many years ago, but I can't find a source without investing a ton more effort, so take this with a grain of salt: IIRC, Plutarch wrote the sayings in intentionally archaic (for his time) Greek. He lived hundreds of years after Sparta had disappeared as a power in the ancient world, so he could have intended it to demonstrate the great antiquity of the quotations, or he could have done so to paint the Spartans as rough-around-the-edges rustics. The translation attempts to mimic this effect by translating the archaic-sounding Greek as archaic-sounding English.