Sharing is Caring
Some people need you to spell things out for them. Like, seriously, you can’t just give them vague instructions and hope for the best. If possible, your orders should be provided in a written document that clarifies exactly what you want them to do. This includes sharing grapes.
When the OP told their partner that they could have half the grapes, the partner did what most reasonable people wouldn’t. Instead of plucking off half the grapes from the stem, they proceeded to eat half of each grape. This half-grape eater wasn’t wrong either. Nobody wants to count grapes and divide that number by two! This solution was much easier, albeit less hygienic.