How does the same displacement at two nodes in a quadrilateral element result in a constant strain overall?

Hi, I've been finding strain in a quadrilateral element using shape functions. I'm a little confused with the interpretation of the strain, though.

When a constant displacement is applied to the upper half of the quadrilateral (nodes 3 and 4) it results in a constant strain across the entire element.

Why does this happen? My code is attached below. The Bmat function defines the B matrix.

clear all
% x & y coordinates of nodes in real space
x = [0.0 0.5 0.5 0.0];
y = [0.0 0.0 1.0 1.0];

% Vector of displacements [u1 v1 u2 v2 u3 v3 u4 v4]
U = [0, 0, -1.5e-4, 0, 1e-3, 1e-3, -1.5e-4, 1e-3]';

% List of coordinates in parent space where strain will be calculated.
xiList = [-1, 1, 1, -1];
etaList = [-1, -1, 1, 1];

% Dummy matrix where strain is to be input
strainList = zeros(3,length(xiList));
% Loop over all of the points listed, calculate the strain, 

% then store it in each column of strain
for point = 1:length(xiList)
    %define xi and eta for this point
    xi = xiList(point);
    eta = etaList(point);
    %calculate B given xi and eta
    B = Bmat_4n(x, y, xi, eta);
    %calculate the 3 strain values for each point required
    strainList(:, point) = B*U;
end

%%
% Plot a contour of displacement in the x direction
figure(1)
patch(x,y, U(1:2:end-1))
title('Displacement in the x Direction')
xlabel('x coordinate')
ylabel('y coordinate')
colorbar()

% Plot a contour of displacement in the y direction
figure(2)
patch(x,y, U(2:2:end))
title('Displacement in the y Direction')
xlabel('x coordinate')
ylabel('y coordinate')
colorbar()

% Plot a contour of strain in the x direction
figure(3)
patch(x, y, strainList(1,:))
title('Strain in the x Direction')
xlabel('x coordinate')
ylabel('y coordinate')
colorbar()

% Plot a contour of strain in the y direction
figure(4)
patch(x, y, strainList(2,:))
title('Strain in the y Direction')
xlabel('x coordinate')
ylabel('y coordinate')
colorbar()
caxis([-0.002 0.002]);

% Plot a contour of shear strain
figure(5)
patch(x, y, strainList(3,:))
title('Shear Strain')
xlabel('x coordinate')
ylabel('y coordinate')
colorbar()