Why do some quadratic graphs have a flat bottom?

Question

Why do some quadratic graphs have a flat bottom? This is concerning sketching the graph on a physical graph. An example of it would be the equation:y=x^2 -5x+4

Answer 1

To illustrate the answer you've been given in the comments (using MATLAB code)

x = 0:0.001:5;
plot(x,x.^2-5*x+4);
x = 0:1:5;
hold on;plot(x,x.^2-5*x+4)