Equivalence point of titration of Sodium carbonate
When the $\ce{Na2CO3}$ is completely neutralized by the $\ce{HCl}$, the solution will be saturated with $\ce{CO2}$, so the pH will be lower than 7 - think of carbonated beverages. Although the $\ce{CO2}$ bubbles out, not all of it bubbles out - not even if stirred. If you had stopped the titration at pH = 7, the pH reduction from 11 would have been partly from $\ce{HCl}$ and partly from $\ce{H2CO3}$.