That's not at all unexpected. Ground from a portable generator is not in any particular way related to the hot or the neutral; the generator output is floating. Usually it will end up floating such that ground is somewhere near "halfway between" the hot and neutral, so 61V is very much in the range of what you'd typically see.
Ground and neutral are at the same voltage (or practically the same voltage) in a house because they are bonded together at the main electrical service panel. If they were not bonded together, they'd generally tend to behave in a similar manner (at least much of the time). It would also be contrary to the electric code for a fixed installation, of course.
