Same Weights, Same Prompt, Different Triage Level

I ran a 4-bit medical-triage model on a laptop GPU and on a CPU. For one patient, the GPU said urgent and the CPU said emergency. Same model file, same prompt, same input. Here's the mechanism and why "validated on hardware X" doesn't mean what you'd hope. I've been building Aegis-MD , a local-first emergency-department triage console. You hand it a structured clinical picture: chief complaint, vitals, age, pain score, a few risk modifiers, and it returns an urgency category on the Australasian Triage Scale (ATS 1–5), where ATS-1 means resuscitate now and ATS-5 means this can wait two hours . The whole thing runs on-device: a quantized MedGemma 4B served through Ollama, a small RAG layer over open guidelines, and a deterministic rule-based floor underneath the model. I never set out to write about floating-point arithmetic. But while running my evaluation set across two machines, I hit a result that stopped me, and the explanation turned out to be more interesting and more current than the textbook answer most people reach for. The setup, and why a 4-bit model Two things about Aegis-MD's design matter for this story. First, it's local by design. Triage data is about as sensitive as data gets, so nothing leaves the machine. The trade-off is that I'm running a small, heavily quantized model: MedGemma 1.5 4B at Q4_K_XL , about 3.4 GB rather than a frontier API. Four-bit weights are the price of running offline on consumer hardware. Second, I tested on two configurations on purpose. The intended deployment is local GPU inference (an RTX 5070 Ti Mobile, 12 GB). But the public demo runs CPU-only on Cloud Run, because GPU instances need a paid quota I don't have. So I ran the same evaluation against both: the GPU build and the CPU build, same model, same code, same prompts. The eval is 17 hand-written cases spanning all five ATS levels, cardiac arrest down to a medical-certificate request. (Seventeen is a smoke test, not a validation; I won't quote a percentage off a sampl