Why do fewer neurons NOT make you dumber?

A blogger I like references a study he can see and I can't. In the Proceedings of the National Academy of Sciences some Columbia University researchers stopped the development of neurons in some rats to see how it affected their performance on Rat IQ tests like maze figgerin'.

In a demonstration of how intelligence seems to work, fewer neurons did not make the rat dumber. Lets let one of my favorite brain bloggers, On the Brain by Dr. Michael Merzenich, Ph.D. explain:

"In most of the brain — including the cerebral cortex and the primary brain centers that support the cortex’s contributions to learning, cognition and memory control — there is little or no neurogenesis in adult brains. Nonetheless, the brain is, by its very nature, a plastic (LEARNING) machine, and the cortex is crucially involved in all memory-guided learning! It learns through changing synaptic weights (strengthening synapses that are engaged in a good ‘try’; weakening those whose actions are irrelevant for it), and NOT because new neurons pop up!"

(Via On the Brain)