[tmz]

Yeah, it's a bit arcane ,particularly since there is debate over what the optimal ratio between HCO3 and CO3 is in variable tank conditions.
The scrubber just let's you move ph up without directly affecting total alkalinity since you are not adding any CO3; your not adding any H either as you would with bicarbonate(HCO3) additions.
Normal seawater at 8.2 has a ratio of around 110 bicarbonate to 20 carbonate if I recall correctly .(Note: 1 meq/l alkalinity is about 50ppm, 2.8 dkh) If you had 130 ppm in your tank the 130ppm wouldn't change unless you added carbonate or bicarbonate.

Yes, the ratio between carbonate and bicarbonate will change in response to ph. Higher ph = less H = more carbonate and less bicarbonate. A higher ratio of carbonate will increase buffering capacity ,which is not the same thing as alkalinity. Carbonate adsorbs 2 H+ protons before becoming carboninc acid while bicarbonate (HCO3) adsorbs only 1.Ph as you know is a measure of the water's ability to neutralize acid.

Our tanks aren't oceans and our homes aren't global air masses .Excess CO2 can easily build up and depress ph via H additions from hydrolized CO2 forming carbonic acid even when alkalinity is high. CO2 can get high in a tank if the air around the tank is high in CO2 as well as from CO2 contributions from the concentrated bio activity in the little droplets we call reef tanks. While the tank will equilibrate with the air eventualy, it takes a bit of time; it's faster with more surface agitation or bubbling like a skimmer. A way to limit CO2 input at least from the air pumped in from the skimmer is a useful way to lower the CO2 level in the tank.

What is optimal ph for a particular aquarium is really the heart of your question,I think?

The scrubber just gives you an ability to raise phin an efficient way without adding alkalinity when you think you should raise it.

Recommended range for ph are 7.8 to 8.5. I'm trying to aim for 8.2( when in doubt I follow mother nature ). 8.2 or 8.3 is also mid point in the range, so risks of dissolving coral skeletons or precipitating calcium carbonate are lessened. However, what the best ph is for various specimens and and systems varying in nutreints and other variables is highly debatable.
The scrubber is just a tool which enables some direct ph control without adding anything to the tank but air with little or no CO2 in it..