Gordon_S,

What you are seeing is Hyperthreading. Carrara sees Hyperthreaded CPUs as essentially 2 CPUs. This gives you ~20-25% increase in rendering speed over the same processor with Hyperthreading turned off. If you had a PC with Intels Extreme 840 (3.2 GHz) dual core with Hyperthreading, you would see 4 render patches during a render on one PC.

The P4s deep predictive branch pipeline is normally a liability for rendering, which is why Intel came up with Hyperthreading. The single core becomes a virtual 2 core processor. Carrara sees this (as most modern 3D software does) and thinks it is running on a dual processor PC or at least multithreaded, which in the past was multi-processor.