Technically, I think that's because a refractometer can't directly read SG. And the problem is the relationship between SG and Plato are not perfectly linear. For the same reason, a hydrometer can't directly read Plato.
So when a brewery gives starting and finishing plato, which instrument gives them that? Refractometers give degrees Brix and hydrometers give specific gravity. What gives plato?
Brix = Plato
Acutely that s not correct.
There is 1.04 correction factor.
I think it has to do with what kind of sugar it is.
I use hydrometer with Plato scale and thermometer with adjustment factor.
I have one for low range (0-9) and one for mid range (8-16)
I should also buy the high range but I do not brew too many beers over 16 Plato.
I like this hydrometer better then refractometer.
I think it is more accurate.
I measure only Last runnings and wort after cooling.