Chunks still loaded behind fog

I was on my single player world in 1.7.4 and was on the render distance 8 when i had left the chunk i was in i was lagging for some reason. I looked behind the fog all the chunks wer loaded still.

I had a check at 1.6.4 and tried it on that version all the chunks unloaded when i left those chunks and had no lag. Can you fix this please