Locate max by clicking on 2D surface

Hello! I try to write some siple code that should find and mark maximum in defining area. I have some code, but I don't understand. why marker is always righter than I click. I will be happy for any proposals.

close all;
f = figure;
s = surf(peaks);
view([0 90]);
shading interp;
GetMousePosition(f,'on');
s.ButtonDownFcn = @get_data_from_clicking;
function get_data_from_clicking(~, ~)
    coords = get(gca, 'CurrentPoint');
    [x,y] = get_xy(coords(1,1), coords(1,2));
    [xx,yy] = find_maximum(x,y,peaks);
    hold on;
    scatter3(xx, yy, 10^4, 'filled');
end
 function [xx,yy] = get_xy(x, y)
    xx = floor(x);
    dx = xx + 0.5;
    if dx - x < 0; xx = dx; end
    yy = floor(y);
end
function [xx,yy] = find_maximum(x,y,mat)
    xx = 0; yy = 0;
    x_start = 2*x - 1;
    y_start = y - 2;
    I = 0;
    for i = 0:2
        for j = 0:2
            if mat(x_start + i, y_start + j) > I
                xx = x_start + i;
                yy = y_start + j;
                I = mat(x_start + i, y_start + j);
            end
        end
    end
end

[Igor Arkhandeev]

I find that find_maximum function changing location of marker.