Tangosol created Coherence, and when Oracle bought Tangosol it applied its usual politics : first, they do not change the numbering scheme (too much stuff to do before, do not scare the users), and then, when things are running smoothly, the next major release enters the magic global numbering scheme 8g, 9g, 10i, 11i, 12c... They did the same when they acquired BEA, for instance (Tuxedo, Weblogic...). I guess Sun products went the same way (even MySQL I believe).
I do not know when exactly Oracle acquired Tangosol, but somewhere in the 3.x series. As usual for software, numbers do not mean much. The first digit is changed only for major releases (3->12), the others are for patches/ "minor" upgrades.