The Mystery of Cerebrum and Cerebellum in a Brain

Our brain has roughly 100 billion neurons, and 1 quadrillion synapses. Cerebrum holds kinda 80% volume of our brain but only with 20% of neurons, while cerebellum is of just 20% volume but with 80% neurons. This is the mystery.

How the brain functions:

• Signals go to senses (eyes, ears, etc)
• Inputs
• Join at back of brain (sensory join)
• Joint Input
• Go to top of brain (concept area) for analyzing
• Processing
• Go to front of brain (motory) for making action
• Action
• Parts of cerebrum may connect to core areas (cerebellum, hippocampus, etc) to store info.
• Storage

I guess the functionalities of those areas in brain are similar to this:

• CPU/RAM: Cerebrum
• Disk: Cerebellum & core areas

The brain can have that kind of 1 quadrillion synapses (5% to 10% of all possible connectivities?) only because it may be similar to artificial neural network that it's no meaning when all neurons are connected in a fully-connected graph (graph theory). A fully-connected graph carries no meanings, just a mess.