A refractometer doesn’t directly measure the density of the liquid, but it does measure how light is bent through the liquid. The fundamental scale of a refractometer is the Refractive Index (RI). As discussed earlier, temperature of the medium and the wavelength of the light do affect the Refractive Index of a substance. To be truly “standardized,” RI measurements with a refractometer should be done at 20 degrees Celsius using sodium light (589.3 nm). On a homebrewing level, that’s not really practical, nor necessary. The presence of dissolved substance(s) in water (i.e., its concentration) also affect(s) the refractive index. The Brix scale is a sucrose concentration scale utilizing standardized settings (sodium light, weight % of sucrose in water, and 20 degrees C). It is a two-component system.
Wort is not a two-component system, nor a true (i.e., homogeneous) solution. Wort is a complex, heterogenous liquid mixture. While most of the dissolved substances in wort are sugar, a significant portion (>5%) is not. Furthermore, sucrose only comprises a small percentage (3-8%) of the sugars, like maltose, maltotriose, glucose, fructose, and other dextrins, which, incidentally, exist in greater percentages and have different relative densities than sucrose. Liquid mixtures that are not homogeneous include colloids, suspensions, and emulsions; wort has features of all three.
Yet, the Brix scale has usefulness in brewing. As home brewers, we should be using the term “apparent Brix” for non-sucrose based liquids like wort as it gives us a measure of the total dissolved solids in the wort which are predominately sugars. It would not be unreasonable for the “total dissolved solids” measurement to decline over time if given a reasonable period of time for substances to stratify or sediment based on their relative density and weight. Refractometer readings taken immediately after sampling permits no time for evaporation or stratification.
Brix RDS (Refractomeric Dissolved Solids) is a measurement of percent by weight of total dissolved solids in solution; and is determined using a refractometer. It is NOT the same thing as Brix degrees, which is a measurement of the mass ratio of dissolved sucrose to water in a liquid and is determined through a specific gravity method, e.g., a hydrometer or a saccharimeter.
Hydrometers aren’t perfectly accurate, either. Given that alcohol is lighter than water, how many home brewers really boil off the alcohol when taking FG readings (to determine real attenuation rather than measured attenuation)?
I just don’t think “evaporation” is causing your readings to be variable with the refractometer.
It would be interesting to see results of the following experiment when you do your next batch: Go ahead and chill your covered samples before taking your refractometer and hydrometer readings to determine OG. Take one reading without disturbing the sample, and then take another refractometer reading after agitating the entire sample. If it is due to sedimentation, your second reading should be higher. Do your hydrometer reading last. As long as your sample remains covered during chilling, evaporation should not be a factor.