I doubt it's seeding Celtic.

More likely imo (as others have suggested) it's people using the "Cash Out" button to green up - that will place a tiny bet on the Greens at 1000 to ensure the outcome is flat green across all selections.

That could have a similar effect Celtic - if you want to lay party A and better value is available by backing parties B, C, D and E then that will happen behind the scenes and money will get matched at 1000 on party E for pennies.