It is historical. Originally it meant salt prepared using twice the amount of acid, so this is preparation of a normal salt, carbonate:
Ca(OH)
2 + H
2CO
3 CaCO
3 + 2H
2O
and this is making of BIcarbonate, using two times more acid:
Ca(OH)
2 + 2H
2CO
3 Ca(HCO
3)
2 + 2H
2O