If you hover over the "Variation with temperature at constant pressure" chart you get a "sources" link, which references National Institute of Standards and Te

If you hover over the "Variation with temperature at constant pressure" chart you get a "sources" link, which references National Institute of Standards and Technology, NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) https://www.nist.gov/srd/refprop https://www.nist.gov/srd/refprop I.e. these are standard reference curves at a fixed temperature. If you click the link on "Hydrogen" you get a page for molecular hydrogen, H2, so not an isotopic mix. Whether that's the right input for this calculation is beyond my technical knowledge, but Wolfram is displaying its parameters quite straightforwardly. To my reading it looks like a quite well-explained calculation, though I can see why you might think otherwise if you hadn't spotted the somewhat-hidden "sources" link.