Difference in titration curves
Maybe your electrode is incorrectly calibrated. But it may be another effect : you should know that at high acidic concentration, the pH values is not obtained by taking the logarithm of the concentration of $\ce{H3O+}$. At high acidic concentration, the concentration should be replaced by the activity, which could be rather different from the concentration. Just for the fun try this experimentation : Take your $\ce{HCl}$ solution with an electrode showing  pH $2.1$. Add an equal volume of saturated $\ce{NaCl}$ solution (which is neither acidic nor basic). You will not believe your eyes by reading your electrode : The pH values goes down to $1$, as if diluting the initial solution produces an increase of concentration !
The activity is sometimes described as the amount of $\ce{H3O+}$ ions divided by the volume of the so-called &quot;free water&quot; (and not by the volume of the solution). In concentrated solutions the &quot;free water&quot; is this small amount of water that is free to move, and not attracted and fixed around the ions. One Liter $1 M$ $\ce{HCl}$, should contain much less that $1$ Liter free water, maybe about $0.1$ Liter free water, or less.