As mentioned by Mash Monster if you brewed to the instructions the gravity should be around 1.040 as you brewed a little short ot may well be a tad higher (but not 1.080). Temperature does have an impact as hydrometers are typic calibrated at 20°C but again you wouldn’t see a huge difference at 23°C.
Your issue could be caused by the hydrometer being defective, or it just bobbing up and down a bit in the liquid making it hard to get an accurate reading, the scale on your basic hydrometer tend not to make things easy. With regards to the three scales, I assume you haVe a 3 scale hydrometer, these are typically specific gravity, Brix (which I had to google to check what it actually is it’s the amount of solids dissolved so in the case of brewing this would be sugar per 100ml) and potential alcohol.
The potential alcohol should be typically ignored this is primarily for wine makers as it assumes the gravity will drop to 1.000. The Brix scale however is interesting as from the looks of things 1.080 SG is roughly 20 on the. Brix scale. This does make me wonder of you hadn‘t quite stirred enough when you took your sample and if it was from the bottom of the fermentor where the malt extract had sunk to you could get an unusually high reading, or you simply misread the scale.
Ultimately I wouldn’t worry too much with kits if you make them to the instructions you will end up with the expected gravity, making it short to 21 litres will have a minimal impact but not enough to bring your gravity up to 1.080. You are likely around about 1.040+/- a few points or so and will likely finish around about 1.008+/- a couple of points so around 4.5-5% alcohol wise.
