Why is 98.3% sulfuric acid used to absorb sulfur trioxide in the contact process, and where does that acid come from?

Why is 98.3% sulfuric acid used to absorb sulfur trioxide in the contact process, and where does that acid come from?
In the contact process, SO3 is absorbed not by water but by 98.3% sulfuric acid to avoid forming acid mist. The absorbing acid is recycled: a plant initially purchases a stock of concentrated acid, which is pumped to the top of an absorption tower. There it reacts with SO3, becoming stronger or even oleum, and returns to a storage tank. Water is added to the tank to keep the concentration at 98.3%, so the acid volume grows. The excess is drawn off as product. Some plants also use 98.3% acid to dry SO2-containing gas; that acid picks up water and drops to about 92.5%, which is then returned to the tank, effectively serving as the water addition step.