A big point of confusion is that it is still taught (at least in the mid-2000's) that STP is defined with respect to 273 K$\pu{273 K}$ and 1 atm$\pu{1 atm}$ of pressure, or 1.01325 bar$\pu{1.01325 bar}$ of pressure, even though IUPAC changed their definition to be with respect to 1 bar$\pu{1 bar}$ of pressure. By using the ideal gas law on the old STP definition, you get that the volume is $\approx 0.0821\cdot 273 \approx 22.4$ liters.