Tea taste changes if watered down after steeping
Water extracts flavor and color compounds from the tea leaves. Water can only hold so much of these compounds before it reaches the saturation point, where it cannot hold any more. Any flavors left in the tea leaves are unable to be extracted by the water. 

If you only add 50% of the water required to extract all the flavor and color from the tea you leave much of the flavor behind in the leaves, and the tea will taste strongest of the compounds that are extracted fastest. When you then add the other 50% of the water the tea will be weaker and taste different than tea that has been steeped in 100% of the water needed to extract all the flavor and color.