That is what I understood as well, though I don't know which VDW radii (enhanced) PISA uses. I will have to dig into its source code to find out, since they aren't listed on the webpage. But PISA's calculation is simple because one gets the same answer with or without hydrogens. In ChimeraX, one can get an SASA for a structure without explicit hydrogens, but it is partially compensated for hydrogens. Alternatively, one can add hydrogens first and then calculate an SASA, which uses a different set of VDW radii and gives a different result from the first calculation. That is why my students are confused. I'm not saying there is something wrong with how ChimeraX handles the calculation. I just want to make sure I understand what is going on here.