Selecting the first node set

<Trade>
	<UnknownName1>
		<Value>
			<enum>A</enum>
			<enum>B</enum>
			<enum>C</enum>
		</Value>
	</UnknownName1>
	<UnknownName2>
		<Value>/Trade//Value[ancestor::Trade]</Value>
	</UnknownName2>
	<Treatment>
		<UnknownName3>
			<Value>
				<enum>D</enum>
				<enum>E</enum>
				<enum>F</enum>
			</Value>
		</UnknownName3>
	</Treatment>
</Trade>

/Trade//Value        - returns all 3 <Value> nodes
/Trade//Value[1]    - also returns all 3 <Value> nodes (why is that ?)
/Trade//Value[2]    - returns nothing (why is that ?)