Here is information from my colleague, @tonya_mauldin, an expert in Control Chart Builder:
"When the specified limits do not equal Avg +/- 3*sigma limits, the Westgard Rules are based on the zones. If the user adds zones they should be able to tell that the Westgard rules are, in fact, using the user-defined limits. In his image, the distance from the center line to the upper control limit is smaller than the distance from the center line to the lower control limit. The zones are calculated using the smallest distance, so for this case, they will be calculated using the distance from the center line to the upper control limit."