That's because the high alkalinity is supplementing what was used since last waterchange. It's not necessarily the base line to run your tank at.
There's reasons to have high alk, but like branching hammer said, (paraphrasing) you're living in the razors edge. It can be more forgiving if your phosphates are high.
It's not near any border unless you run an ultra low nutrient system which isn't even popular anymore. Arguably corals may grow faster at higher alk if it does anything at all. You don't need high nitrate or phosphate to maintain higher alk values, just not ulns numbers.


