Why are FMatrix's multiplied this way?

From the doc FMatrix | Unreal Engine Documentation

4x4 matrix of floating point values. Matrix-matrix multiplication happens with a pre-multiple of the transpose in other words, Res = Mat1.operator*(Mat2) means Res = Mat2^T * Mat1, as opposed to Res = Mat1 * Mat2. Matrix

My math-fu is weak. Why is this matrix multiplication done with a pre-multiple of the tranpose? Is this for optimization purposes?