There’s some subtype A of ChipValue (which may be ChipsValue itself, but could be a real subtype) which is provided upon instantiation of ChipsPane. Now inside ChipsPane you assume that A =:= ChipValue. Consider MyChipValue extends ChipValue and new ChipsPane[MyChipValue], though - the assumption doesn’t hold.
A can be any subtype of ChipValue (or ChipValue itself), but for a given instance of ChipsPane it is a specific subtype of ChipValue (or ChipValue itself) that is provided upon instantiation. For new ChipsPane[MyChipValue], A is bound to MyChipValue, and chip clearly is not of type MyChipValue.