Blizzard has clarified that the extra 500 Traders Tender received by players this month is an intended but unlisted reward from the Trading Post Enthusiast achievement for completing the monthly trading post bonus twelve times.
This helps explain why some players did not receive the currency, which had led to speculation that the extra 500 was the result of a bug or had been hotfixed out earlier this morning. Players that have so far completed each month should be awarded the extra 500, while those who missed a month should catch up over time.