Lyngsat does not tell you where a satellite is. It predicts where it would be based on where the satellite was at a particular time and on how it was moving.
An analogy.
A ship leaves New York harbor heading due east at 15 knots on 10/23/2004 at 13:00 (1pm). Where is the ship now?
If the ship continues on the same course at the same speed you can predict where it is now fairly accurately.
However, if the ship changes it's course and or speed your prediction will be off. Instead of being halfway across the Atlantic heading toward Europe it is in the Caribbean heading toward the Panama Canal.
Most satellites operate so that their course and speed stay the same and their position can be accurately predicted. AMC-15 is/was being maneuvered, changing course and speed in order to reach a specific location. As such trying to predict where it is now based on it's course and speed as of 3 days ago will not provide an accurate answer.