Originally Posted by ubermensch
Sorry, I meant 20 L of liquids total, in which there's 4 kg of honey. That's in a 25 L bucket. That mix gave me a 1.079 reading. I added about 1,5 L of water and the SG dropped to 1.076. The honey was first dissolved in 10 L of water at ~55°C. That worked quite well, I would eliminate stratification.
Ok, so the hydrometer is fine and stratification is not the cause.
Yet, there still is something not quite right. Using your new recipe information (20L of water & 4 kg of honey) yields a must whose SG is 1.052 and total volume is 22.8 L (6.03 gal). Adding another 1.5 L of water would lower the SG to 1.049.
FWIW, these values are not approximations. They are based on sound solution concentration principles that I validate each time I make mead. Typically, the accuracy is within 0.005 points - deviations are usually due to honey addition error (mine).
An OG variation of 27 points (1.052 vs. 1.079) is considerable. Unless that difference is not a concern for you, I'd suggest it would be benefical to discover the reason for it before trying to make something where that difference was more critical to the outcome.
As a matter of interest, I found that 6.6 kg of honey added to 20L of water is needed to produce an OG of 1.079...