Why bwboundaries with roipoly change the size of a rectangle?

Dear all,

Could anyone tell me why the size of rectangle is changed?

x = [4 10 10 4 4];
y = [4 4 10 10 4];
BW = poly2mask(x,y,20,20); %binary image that contains a small rectangle
B = bwboundaries(BW);
b = B{1};
X = b(:, 1);
Y = b(:, 2);
hpoly = roipoly(BW,Y,X);
figure, imshow(BW)
figure, imshow(hpoly)

Any idea will be appreciated.

Best, Meshoo