I see it this way: An individual has the capacity to control a certain amount of magical energy, Mana, just as one can understand things or have skills up to a certain level. Your ability with a sword is increased through training and experience. Your ability to handle wounds is increased by learning to roll with punches and minimise the long- and short-term effects of wounds. Your ability to handle magic is raised through experience in casting and training of mental control.
Your mana level reflects how much magical energy you can manipulate. If you are in a Maths exam, you can only calculate so many equations before your brain says "No more!" Some people can answer more questions, others less, depending on their inherent nature and studies. Mana potions are like a clensing; they clear your mind of magical mental blocks. It's like when you drink coffee (or what ever your beverage of choice is) at work to give you that extra kick.
Items enchanted to benefit the wearer are helping to take that burden away from the body. A sword that increases your Offense or Agility is probably lighter and easier to wield. Armour that increases your defence or hit-points is designed to take blows more effectively. A staff that increases your Intelligence or Mana is helping channel the magical energies. By acting as a Mana Rod (as opposed to a Lightning Rod), the staff can act as an attractor of untapped Mana. Combining that with your minds ability to cast spells, you can use more Mana because the staff takes some of the burden of chanelling.
How Strength is increased, I'm not so sure. That's just plain magic.
I believe that you cannot channel more Mana than you are able, just as you can only do so much math before you start halucinating. You have a maximum Mana level for this reason. If somehow, more magical energy was pumped into you, I believe that your body and mind would have a similar effect to a drug overdose. You could handle chanelling a certain amount of magical energy, just as you can drink so much alcohol, but too much is poisonous, just like alcohol.
This is my take on Mana and all things mathmatical in Divine Divinity, and any other RPG for that matter.
