I get that there would be a correlation, for all the reasons you mention, but that it's stronger than others is odd to me.

I wonder if there's something here like people who cook more or garden have stronger grips, so it's picking up some difference in food intake as well.

You think so? I think it makes sense. 

Someone with a stronger grip is more likely to have more muscle mass and to engage in activities that increase strength which is healthy.