OpenGL the direction of translation is not right after rotation - opengl

Question

I'm writing a simple demo with the fixed pipeline mode in OpenGL
First I write some codes that will draw a rectangle, with top-left point (x1,y1), and buttom-right point (x2,y2)
// Drawing here
glPushMatrix();
// Perform rotation first
glTranslatef(x_cent, y_cent, 0.0f);
glRotatef(angle, 0.0f, 0.0f, 1.0f);
glScalef(scale, scale, 1.0f);
// Set origin back to the screen center with the rotation set above
glTranslatef(-x_cent, -y_cent, 0.0f);
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
glBegin(GL_QUADS);
{
glVertex2f(x1, y2);
glVertex2f(x2, y2);
glVertex2f(x2, y1);
glVertex2f(x1, y1);
}
glEnd();
// End of drawing
glPopMatrix();
After I tweak the value of the angle with arrow keys, say 45 degrees counter-clockwise, and now the rectangle will rotate 45 degrees counter-clockwise. Then I translate it with arrow keys, it moves in the direction of up, down, left, and right. Everything works fine.
So I write anoter part of code that will draw a general polygon. It's similar to the code above. I simply draw lines between each pair of points
for (int i = 0; i < p_size; i++)
{
float x1 = point[i].x, x2 = point[(i + 1) % p_size].x;
float y1 = point[i].y, y2 = point[(i + 1) % p_size].y;
x1 = ((x1 + 0.5f) / winWidth) * 2.0f - 1.0f;
x2 = ((x2 + 0.5f) / winWidth) * 2.0f - 1.0f;
y1 = ((-y1 + 0.5f) / winHeight) * 2.0f + 1.0f;
y2 = ((-y2 + 0.5f) / winHeight) * 2.0f + 1.0f;
// Set the curent coord origin to this poly's center
float x_cent = (left + right) / 2.0, y_cent = (top + down) / 2.0;
float ratio = winWidth / winHeight;
if (ratio > 1)
{
x1 *= ratio;
x2 *= ratio;
}
else //if (ratio < 1)
{
y1 /= ratio;
y2 /= ratio;
}
// Drawing here
glPushMatrix();
// Perform rotation first
glTranslatef(x_cent, y_cent, 0.0f);
glRotatef(angle, 0.0f, 0.0f, 1.0f);
glScalef(scale, scale, 1.0f);
// Set origin back to the screen center with the rotation set above
glTranslatef(-x_cent, -y_cent, 0.0f);
glBegin(GL_LINES);
{
glVertex2f(x1, y1);
glVertex2f(x2, y2);
}
glEnd();
// End of drawing
glPopMatrix();
}
I rotate the polygon with arrow keys, that works fine, say also 45 degrees counter-clockwise. But after the rotation, when I translate it with up key, it moves not up but in the direction of 45 degrees.
Bbut why can the rectangle translate normally in the 4 directions after rotation? These two part of codes are almost the same, why is this happening...