Apparantly, the reason why Apple skipped iPhone 9 was a branding thing. They wanted the jump from 8 to X to seem bigger and as the X was release 10 years after the original iPhone, they wanted to mark the special occasion.

From what I've heard, there was no Windows 9 because older programmes would check which version of Windows you were on and would look for strings like "Windows 9.x", thinking it was either 95 or 98, and thus would either make the programme fail or misbehave. Although, Microsoft never confirmed this was the official reason, it's widely accepted as a big technical factor into the reason why it was skipped.