Its because supermarket freezers are set at a constant temperature daily, and very rarely go below -18.
Your home freezer is set to -24 and makes things harder when frozen, thats why when you buy Magnum ice lollies the ice cream is just right straight from the supermarket freezer, but if left in a home freezer over night the ice cream is a lot harder.
It does, yes, have a lot to do with the ice cubes melting a little as well. Take a cool bag to the shops with you and stick your freezer stuff in that. Works wonders.