I've been building a tool that monitors how brands show up in ChatGPT, Gemini, Grok, and Perplexity answers. Most "GEO playbooks" I read assume the four LLMs behave roughly the same — same internet, same authoritative sources, optimize once, ship everywhere. Wanted to test that assumption with actual data. So I sampled the same 5 prompts across all four models, four times each over 18 hours, and pulled every cited URL. 700 citations later, the four models barely overlap. The intersection of all four top-10 lists is exactly one domain ( visible.seranking.com ). For top-10 territory — the most important sources — each LLM is reading a fundamentally different internet. Here's what I found, the methodology, and what it means for anyone trying to optimize for LLM visibility.…