Calculation tracking difference

Hello, I'm looking at the total costs of ETFs and the TD plays a significant role there. Now I like to look at extra ETFs to see what the costs/performance are. Now I don't fully understand how extraETF calculates the tracking difference. The normal calculation is explained (TD=Development ETF - Development Index) And then their own calculation (TD=Development Index - Development ETF) According to their formality, a negative TD al