Real attenuation corrects apparent attenuation for a genuine measurement artifact: dissolved alcohol reads lower on a hydrometer than the sugar it replaced, so a straight OG/FG calculation slightly overstates how much of the wort's original extract actually fermented out. Real attenuation applies the Balling real-extract formula — working in degrees Plato rather than specific gravity directly — to correct for that effect, and is always a few percentage points lower than apparent attenuation for the same pair of readings.
Why brewing software and yeast labs often disagree slightly
A yeast manufacturer's published attenuation figure is usually apparent attenuation, matching what most brewers calculate by hand — but some technical brewing texts and lab reports use real attenuation instead, which can make a strain look less attenuative on paper than its apparent-attenuation reputation would suggest. Knowing which figure a given source is reporting avoids a genuinely common source of confusion when comparing numbers across different references.
Where to get both numbers without doing the Balling math yourself
The ABV calculator computes real attenuation automatically alongside apparent attenuation from the same OG and FG inputs, using the same Balling formula described above — there's no need to convert to Plato and run the correction by hand.