How to calculate the mse in the "bboxOffsetLoss" function in the official list yolov3?

When I read the official document to detect vehicles with Yolov3, the mse in the "bboxOffsetLoss" function does not know how to calculate it. Looking at the syntax of using mse, I still don't understand.

function boxLoss = bboxOffsetLoss(boxPredCell, boxDeltaTarget, boxMaskTarget, boxErrorScaleTarget)
% Mean squared error for bounding box position.
lossX = sum(cellfun(@(a,b,c,d) mse(a.*c.*d,b.*c.*d),boxPredCell(:,1),boxDeltaTarget(:,1),boxMaskTarget(:,1),boxErrorScaleTarget));
lossY = sum(cellfun(@(a,b,c,d) mse(a.*c.*d,b.*c.*d),boxPredCell(:,2),boxDeltaTarget(:,2),boxMaskTarget(:,1),boxErrorScaleTarget));
lossW = sum(cellfun(@(a,b,c,d) mse(a.*c.*d,b.*c.*d),boxPredCell(:,3),boxDeltaTarget(:,3),boxMaskTarget(:,1),boxErrorScaleTarget));
lossH = sum(cellfun(@(a,b,c,d) mse(a.*c.*d,b.*c.*d),boxPredCell(:,4),boxDeltaTarget(:,4),boxMaskTarget(:,1),boxErrorScaleTarget));
boxLoss = lossX+lossY+lossW+lossH;
end

"mse" ,Mean squared normalized error performance function?

>>mse(3,5)

ans =

4

>> mse(3,5,1)

ans =

4

>> mse(3,5,2)

ans =

4

>> mse(3,5,2,3)

ans =

4

>> mse(3,5,2,3,10)

ans =

4

Why is this result?