There is a difference between "dissolved CO2" and carbonic acid. When CO2 is
added to water it initially forms a loosely hydrated species denoted CO2(aq).
This dissolved molecular CO2 reacts _slowly_ with water to form carbonic acid,
H2CO3(aq).
CO2(aq) + H2O = H2CO3(aq)
Furthermore, this kinetically slow reaction does not go to completion. At
equilibrium, only a small fraction (ca. 0.2%) is actually converted to
carbonic acid. Most of the CO2 remains as solvated molecular CO2. In fact,
the pKa most often reported for carbonic acid (pK1 = 6.38) is not really the
true pKa of carbonic acid. Rather, it is the pKa of the equilibrium mixture
of CO2(aq) and carbonic acid. Carbonic acid itself is actually a much
stronger acid than this, with a true pK1 value of 3.58.