2006 SV6 Runs out of gas early now?

Ok, my wife has a habit of letting her car get too low on gas... testing the limits to see if she can make it another 10 miles with it flat on E and usually looses the battle. I've tried to tell her for years... just fill it up at a 1/4 tank. Costs the same in the long run. But none the less, she runs it low all the time.

Well a few days ago, I was using her car. It was below 1/4 a tank. But I was feeling lazy so I didn't fill it up. It was at about little over 1/8 tank but definitely not on E (the low fuel light hadn't came on yet) and the car stalled. Called my wife and told her I think the car is out of gas even though the gauge doesn't read E and the low fuel light isn't on. Well, she brought gas and the car started up. It just happened to my wife again. She just ran out of gas at 1/4 a tank. Emergency response team showed up and put gas in and it started. So, the car is running out of gas early. What causes this?