Sweat is chemically different from plain water - it carries salts and oils that plain rain resistance is not always tested against - which is why a pair of earbuds rated fine for light rain can still fail from months of heavy gym use even with the same numeric IP rating.
Most workout-focused earbuds only advertise the second digit (the liquid resistance number) and skip the first digit (dust/particle resistance) entirely, since dust ingress is not the concern for gym use - IPX4 is the realistic minimum for genuine sweat resistance, meaning splashing water from any direction, which covers normal sweat dripping during a workout.
What the rating does not cover is long-term material degradation. Silicone ear tips and mesh grilles absorb salt residue over repeated workouts, and manufacturers generally test the IPX rating on a brand-new unit, not one that has already absorbed months of dried sweat into its seals and vents - which is why washing sweat off earbuds after each session (rather than letting it dry in place) meaningfully extends how long the actual water resistance holds up versus the rated spec.
Sweat composition itself also varies significantly between people and workout intensity, which is part of why some users report earlier connector corrosion than others despite owning the identical model.
CairoVolt's workout earbuds page lists current sweat-resistant models with their actual tested IPX ratings.