Low graphics=x ray vision?

I love crossfire, but with this failpc thats over 8 years old, the fps gets quite low. So i changed all the in-game settings to the lowest possible, and i went into my video cards control panel (nvidia) and changed the settings to "high performance".

After i did this, i noticed something. This doesnt happen all the time, but sometimes im able to see through wall. This only works if there right next to a wall, but it does happen. Some times im able to see them before they take the corner. So whats this all about?