What's going on here... (use browser zoom to really see)

If we draw a triangle that is 1 pixel wide, it can fit "between" the pixels.

As the triangle moves from right to left, the part of it that is at the pixel center gets bigger, so the triangle appears to grow.

The yellow triangle is 1 pixel wide. The green triangle is a "zoom in" showing where the center of the pixel is. The blue stack is 5 pixels wide, but exhibits similar artifacts.