Lead Acid AGM battery testing - Why am I testing it this way?
You could do the entire discharge at the lower rate but then it would take a lot more time.
When discharging at a high rate the voltage may collapse prematurely so the battery seems discharged before it really is.
By combining the two discharge rates you can discharge the battery in a reasonable time but be confident that it is fully discharged.
The lead-acid chemistry is somewhat unusual among rechargeable batteries in that the electrolyte is consumed in the discharge process. Being converted to water with the sulfate ions combining with the lead plate to form lead sulfate.
As a result at high rates of discharge, the electrolyte within the plates becomes progressively more dilute, the discharge for that portion of the plate slows down and the cell voltage drops. At lower rates of discharge, fresh electrolyte can diffuse into the plate fast enough to support the chemical reactions and the discharge can continue further. This is the meaning of the last sentence of your question.
Peukerts's Law