I thought your question was interesting so I fished around a bit, but it looks like most sites including business/finance related sites are treating this number as they treat all year-on-year quarterly earnings announcements. If the announcement used some novel method of calculation I would think they would mention that.

That being said, it's kind of missing the forest for the trees. On the order of priorities for the people "calculating" this data, I imagine accuracy and transparency were pretty far down the list.