The Dewalt 20v battery, uses 5 cells in series.
All other 18v battery packs, use 5 cells in series.
The difference?... nominal voltage vs closer to full charge voltage and the different rules for marketing such products in different regions.
My new Ridgids batteries, warm off charger are reading 20.38v.
4.2v x 5 is 21v.
3.6v x 5 is 18v.
The actual cells used in the battery pack, dictate how much voltage the battery can maintain when powering loads.
Beware of marketing.