About GLfloat accumulate

I practiced NEHE Lesson7 and Lesson9.
In Lesson7, I found that when it roted by x or y for a long time about 10 munites, the cube didn’t look like a cure for some face became smaller and some face became bigger. And then I press left or right to adjust the roted angle. And the cube is normal.

In Lesson9, it’s the same Lesson7. After for a long time, the stars roted not regulerly.

I don’t know why. I doubted that it may be the “GLfloat accumulate” problem. Hope to give me some suggestions about it.