I suspect that the reason is the distance and location of the Sheraton West Lake. The hotel as you said is relatively far from downtown. In addition, the taxi driver will have a lesser chance to pick up a fare on his drive back to Hanoi. If you notice the street that leads to Hanoi, it is situated on top what used to be a dike protecting Hanoi from the flood of the Red River. Home and businesses on either of that street (Au Co street) are for the most part situated at a lower elevation, and not directly accessible from Au Co. Tougher to find taxi fares in this area, in my opinion.