Presumably all of this is explaind by updated E5 chips not yet officially announced or shown on the Intel web site*—
Apple’s stated “up to 12 cores” is odd, because the 8 core members of the Xeon E5 family offer the best feature combinations (8+8=16 cores for a dual-CPU system). But a max of 12 cores implies either a single 12-core chip or two 6-core chips. Apple’s sneak peak pictures show only a single CPU on the unit. Could a 12-core CPU be in the works?
My guess is one CPU chip, but with a choice of 4, 6, or 12 cores. This would fit with the sneak preview images, which show one CPU slot and only 4 memory slots (no apparent room for more memory slots and probably another CPU would be too much heat on one “panel” of the three.
Also odd is that Intel lists the E5 CPUs as supporting no faster than 1600 MHz memory. Which suggests some as yet unannounced CPU update waiting in the wings, but maybe this is known (I don’t follow CPU intricacies).
A final incomfortable thought is that the fastest CPUs might be off limits due to TDP (total power dissipation of 95W, 115W or 135W for various existing CPU models).
The Mac Pro is likely to be delayed until late fall given these inconsistencies (waiting for newer CPUs to arrive). See the leaked roadmap.
* Xeon E5-2600 or “Ivy Bridge-EP” family, or possibly E5-4600 (Ivy Bridge-EP 45) and the E5-2400 (Ivy Bridge-EN).