Because the charged device only supports 18w charging, or the charging cable does not support 40w fast charging.
Well-known fast charging accessories brand Anker has released a number of mini charging, including 18W Nano, 20W Nano, 20W core charging, 30W super charge, 45W super charge, 65W super charge, 65W 2C1A super charge, On the eve of Double 11, Anker Mini charger series has another new product debut - Anker 40W dual mini charger. Made of crystal clear double-layer process shell, it supports 40W high-power PD fast charge output, has two output interfaces, supports dual device fast charge at the same time, and maintains the compact and portable characteristics of Nano series.
It may be that the charging data cable does not support 40w fast charge, or the charging device only supports 18w fast charge.