voltage differential vs max input of regulator?
For the adjustable version of the LM1084, the input/output differential rating is needed because the regulator's only connection to Ground is through the voltage adjusting resistor network.

Yes, the variable output version could be used with a 40 volt input to provide a 24 volt output.

Quoting an input/output differential for the fixed voltage versions seems a bit odd to me.  The LM78xx family of fixed voltage regulators show Input Voltage ratings rather than in/out differential.

The LM317 regulator also has an input/output differential specification.