We know that Kobo employees read these forums. Maybe they decided to randomize the top margin to start flame wars ... you know, for entertainment.

More seriously though: maybe it's the book, settings, or device (Glo vs. Aura) because I wasn't overstating the difference. Even at my default settings, I was getting an extra line of text (larger font and margins). Reducing margins, line spacing, and font size to try to figure out the actual edges of the text display area gave a reasonably clear boundary at half of the top bar.