I really don't understand why people (and media as well) keep saying that BTC reached a new ATH. According to CoinMarketCap, BTC's ATH was at
20.089$ and this event happened on December 17th, 2017.
What happened recently, on December 1st, was BTC reaching a value very cloae to its ATH, but not a new ATH.
You are absolutely right. The justification being given is that last time the ATH level was not achieved in many of the exchanges. While in one or two exchanges it breached the $20,000 level, the prices in other exchanges remained much lower than that. Also, the peak prices could not be sustained for long. Within a few months, Bitcoin was trading at four-digit prices.