Please explain this with more words. I don't know what a double insulated appliance is. Or shorted to which casing, the generator, or the appliance.
Sorry, I was a little confusing. I sort of crossed up floating neutral and double insulated in the same paragraph.
In a modern polarized plug (narrow side and wide side) in a structure, the neutral (wide side) is at zero volts (earthed in electrical parlance). This is due to it actually being grounded via a ground rod.
In a generator, there may be 120v between hot and neutral, but neutral may NOT be at zero volts relative to the earth/ground/your skin.
Given that some things connect the neutral and the frame/casing/etc of the appliance/tool/etc to neutral... that means you can get a shock. It also means the casing is not at zero volts (relative to earth/ground/your skin) to 'absorb' any short circuits.