There could be two reasons I can think of for your experience with one side being softer than the other.
The first is that there will be a break in period for any new mattress or sleeping system as the mattress loses any of it’s “false firmness” and the cover stretches and loosens a little and the materials in the mattress settle (see post #3 here) so it’s certainly possible that one side could feel firmer than the other if it has been broken in more than the other.
The mattress also contains 3" of memory foam which will soften to some degree with heat, humidity, and continuous pressure so it’s also possible that the memory foam on the side that your husband was testing had softened more under his greater weight (or possibly body heat) than the side you were testing so it felt softer when you switched sides. In general it can be a good idea to spend a little longer on a mattress that contains thicker layers of memory foam so that the memory foam has a chance to soften with heat and pressure so that what you “feel” will be more representative of how it will feel when you sleep on it over longer periods of time.