My somewhat loose understanding of it (in Unixy systems) is:
The type of shell that you’re using has an init script, e.g. ~/.bashrc which sets up custom paths. This script is run when opening a terminal window or tab.
But it wouldn’t run when launching the SC interpreter process.
I haven’t tested closely enough to know that this is definitely the case or not, but if true, it would explain why the system can find executables when running a command in a terminal window but not otherwise: if the command’s executable is not in the default system PATH, but the location of the executable is added to the PATH in the init script, then the init script would help find the executable for runInTerminal but not unixCmd.
Consider that an educated guess on my part but I wouldn’t be surprised if that explains the case in the original post.
hjh