IEEE-754 only mandates correct rounding for +-*/ and sqrt. Transcendentals (sin/cos/exp/log/pow) are explicitly allowed to vary in the last few ULPs, and glibc, musl, MSVC, and Intel SVML all do. PID is just basic ops, so libm divergence doesn't hit there, but motor vector control or sensor linearization touches these functions every cycle and small disagreements compound. Two firmware revisions can have zero source diff and still drift in production. The only thing that changed was the linked libm. It actually shows up in Payne-Hanek argument reduction and at the worst table-maker's-dilemma boundaries. Probably why safety-critical guidance pins a specific libm build instead of just "IEEE-754 compliant".