HP49-50G: gallonUK conversion into l

[Gil]

HP49-50G: gallonUK conversion into liters

Where from does give HP50G
that 1GALUK is '4.546092_l',

when the official value
is 4.54609_l?

Of course, there existed a prior definition, but it was
4.546 091 879 liters.

Then why this unexplained value or rounding?

Thanks for your insight, as HP is or was almost always "right" in its choice.