This is the reply when I asked Den's Tea:
Japanese teas are made through more processes of kneading and rolling
than Chinese teas. Through those processes, the fibers are more broken
and the ingredients are extracted very fast. At 2nd brewing, the tea
leaves are fully opened and if you steep long, the taste become bitter.
(The ingredients or flavor are extracted even faster than the 1st
More ingredients are still left with Chinese tea after 1st brewing
because many Chinese teas are tigtly rolled and its fibers are not that
broken like Japanese teas. Also Chinese green teas are less bitter than
Japanese teas, so you may want to steep longer to get more flavor, though
the bitterness wouldn't come a lot.
Hope this information answers your question.