The 8 and 8+ are really a 7s and 7s+... Not sure why they didn't just do it that way to avoid confusion.
Because then they would've had to skip two numbers to get to X and they really wanted to call the 2017 phone "ten" because it's the 10th anniversary of the iPhone (and since Windows skipped 9, skipping 9 seems to be acceptable).
From now on they're probably just going to have X2, X3, X4, etc. and never reach 11 just like with macOS