It seems that the Activiti tool is putting x and y attributes relative to itself rather than relative to the BPMNPlane. For example, I attach an example drawn in Activiti and then how it shows up in another tool.
The relevant reference in the BPMN spec is on p377
The bounds of BPMNLabel are always relative to the containing plane�s origin point. Note that the bounds� x and y coordinates are the position of the upper left corner of the label (relative to the upper left corner of the plane).