"When RuneScape 2 was released, the experience required for level 10 in any skill was increased slightly. This meant that characters that had not gained Constitution experience in RuneScape Classic were brought to RuneScape 2 with only level 9 Hitpoints. Despite this, the players were still level 3 combat."
Runescape Wikia
tl:dr If you didn't train in HP and transferred your account from Runescape Classic -> Runescape 2 you would have 9 Hp.