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

A reason to do this would be to get a quicker assessment of the charge capacity at a constant 5A drain. Another may be because that more closely simulated the actual battery drain on the Llama farm 75 years ago.

All batteries tend to have a characteristic open-circuit voltage to which they'll recover under no load. For some chemistries (NiCd, dry cells), this no-load voltage is pretty constant. For some chemistries -- notably lead-acid -- this no-load voltage diminishes with diminishing charge (I'm looking at a chart* that's claiming 2.2V at full charge down to about 1.6V at nearly dead).

Beyond that ultimate no-load current, most cells have an equivalent circuit that looks something like this:

^{simulate this circuit – Schematic created using CircuitLab}

For quick discharges, you "see" the last resistor in the chain, for longer discharges you "see" resistors further back, etc.

For all cells, as the cell gets discharged, the apparent resistances of all the resistors goes up -- each time you discharge the cell, the voltage will go lower and it'll take longer to recover to the open-circuit voltage.

The reason for the above behavior is that as the battery discharges, the ions that conduct electricity inside of it either have more "dead battery" to go through (in the case of NiCd cells), or the electrolyte gets depleted and becomes less conductive (in the case of lead-acid cells).

So whatever they're doing with that test, it has something to do with that behavior of the cells.

* "Rechargeable Batteries Applications Handbook", Technical Staff at Gates Energy Products, Butterworth-Heinemann 1992.