I don't know!
Hope that helps....
But more seriously, the ally kettle probably has more thermal mass than the placky leccy kettle so the water continues to boil briefly as you lift it off the gas to pour, with heat energy from the ally still transferring into the water while you pour and keeping it right on boiling. While the placky kettle switches off and starts to cool immediately before you pick it up and start to pour.
And another idea. Mebbe the placky kettle thermostat turns it orf at say 96c, rather than 100c. A 'safety feature' that is just a bloody nuisance perhaps, like so many of 'em!