It shows that at the same positive AoA, a flat plate generates less lift compared to the NACA airfoils.
From an arbitrary guess, the lift force generated should at least be the same for both
Your guess is right, a flat plate develops exactly the same lift as any other airfoil (at least until stall begins). So, why that difference in the plot?
The AoA is measured in respect to the geometrical line connecting the leading edge with the trailing edge of the airfoil. By an aerodynamic point of view this line has no special meaning. So why do we use anyway this line to plot the aerodynamic coefficients? Because this line is easy to trace and unmistakable and therefore makes things easy to compare (actually this geometric line does posses an aerodynamic meaning but only for symmetric airfoils since it coincides with the line of zero lift for obvious reason of symmetry).
Now, the NACA airfoils of the 44xy series considered in your picture is a cambered one i.e. an airfoil generating lift already at negative AoA (some -5° in that plot - but remember that this -5° is just a geometrical convention and has no aerodynamic meaning). Anyway a flat plate is not cambered rather symmetrical by definition and therefore its lift coefficient is always 0 at 0° AoA: this comparison is unfair! Indeed, if you shifted the plot of the 4412 to the right so that it also starts by 0 lift at 0°, then you would get an almost perfect overlap with the coefficient of the flat plate.
Or... we can simply make a fair comparison from the beginning and use another symmetrical airfoil like any of the NACA 00xy series, for example the ubiquitous NACA 0012 which has the following coefficients:
Here (blue line) you can see that it develops for example a $C_l$ of 0.5 at 5°, just like the flat plate in your plot. And, being symmetrical, it obviously also goes through the point of 0 lift at 0°, again just like the flat plate.
Ok, but if a flat plate generates the same lift as any other symmetrical airfoil why the horizontal tailplane of a jetliner is built around a NACA 0012 and not a simple flat plate? Because of the stall characteristics: a flat plate begins to stall already at around 5 to 10° while the NACA 0012 at 22° and that's the most important improvement introduced by an airfoil in respect to a simple flat plate.