Cannot set static final property

On a static final list property, add can be called but not +=. On a static list property both operators work fine.

groovy> class Globals

{ groovy> static final lst = [] groovy> static lst2 = [] groovy> }

groovy> Globals.lst2.add(1)
groovy> Globals.lst.add(1)
groovy> Globals.lst2 += 2
groovy> Globals.lst += 2

Exception thrown: groovy.lang.ReadOnlyPropertyException: Cannot set readonly property: lst for class: Globals

groovy.lang.ReadOnlyPropertyException: Cannot set readonly property: lst for class: Globals
at Script8.run(Script8:11)