Simple.
9 isn't prime. 3, 5, and 7 are prime. Any consumer-oriented CPU that falls above the i7 line will be i11.
Real answer, though: Intel's naming convention makes no sense, and it has fallen apart since the release of Ivy Bridge.
Nehalem and Gulftown (die shrink of Nehalem) represented the "first generation" of Core CPUs.
Sandy Bridge represented the "second generation" of Core CPUs.
Ivy Bridge (die shrink of Sandy Bridge) and Sandy Bridge-E represented the "third generation" of Core CPUs.
Haswell and Ivy Bridge-E represent the "fourth generation" of Core CPUs.
Why does a die-shrink constitute a new generation? Why is a CPU built on the Sandy Bridge architecture (Ivy Bridge-E) considered part of the same generation as Haswell?