Yeah I'm scratching my head as to how this could happen. This isn't some 0.01 km/h over the pit speed limit "technically a penalty" type thing, 1.5kg is significant.
It has to be the new floor, now that I think of it. They had the new floor on Friday, but it preformed poorly so they switched back to the old on Saturday. Maybe the new floor is 1.5kg heavier, and George's side of the garage forgot to adjust the ballast accordingly after they switched back?
Weren't they both running the new floor? That would mean that they both should be underweight?
I suspect they mucked up the driver ballast, minimum driver weight is 80kg, so if you weigh less than that the team have to add ballast. I've heard that they got the ballast the wrong way round between the two cars and Lewis weighs more so his ballast is less...