What "accuracy" means for a mechanical watch
The published accuracy spec for a mechanical watch is the average daily rate measured across multiple positions and temperatures. A "-4/+6" rating means the watch may lose up to 4 seconds or gain up to 6 seconds per 24-hour day, averaged across 5 positions (dial up, dial down, crown left/right/up) and 3 temperatures (8°C, 23°C, 38°C). The actual daily error you observe on the wrist depends on your activity, watch position when sleeping, and ambient temperature.
The accuracy tiers
There are roughly five tiers in modern mechanical watchmaking. Untreated movements (the base ETA 2824 or Sellita SW200) typically run ±15-25 sec/day from the factory. Regulated to chronometer-grade (most top-spec ETA / Sellita) hits -4/+6 without certification. COSC certified tests every individual movement and certifies -4/+6. In-house chronometer (Rolex Superlative -2/+2, Omega Master Chronometer 0/+5 with anti-magnetism, Patek Philippe Seal -3/+2) is the modern luxury tier. Observatory-tested (historical Geneva, Neuchâtel observatory trials) was the pre-COSC peak; rare today.
Why mechanical watches drift
Mechanical watches keep time by counting oscillations of a balance wheel, which swings under the action of a hairspring (a tiny coiled spring). Theoretically the swing rate is fixed; in reality, gravity, magnetism, temperature, age, and lubricant viscosity all subtly affect the rate. Position is the biggest factor: the balance wheel's rate differs by 5-15 seconds per day between dial-up and crown-up because gravity changes the friction load on the pivot. The tourbillon was Breguet's 1801 attempt to average position errors out by rotating the entire balance assembly.
What you can expect on the wrist
A new COSC-certified watch worn daily, fully wound, will typically run +1 to +5 seconds per day for the first few years (most watches are deliberately regulated slightly fast so they stay positive over time). After 5-7 years, oils dry, parts wear, and the watch starts running 5-15 sec/day off. A service brings it back to factory spec. A 50-year-old vintage watch in good order can still hit chronometer-grade accuracy with proper service.
Beyond chronometer-grade
For accuracy beyond chronometer-grade, look at METAS Master Chronometer (Omega): tested at the watch level (not just movement) at 0/+5 sec/day plus 15,000 gauss anti-magnetism. Or Seiko Spring Drive: a hybrid mechanical-quartz mechanism (mechanical mainspring driving a quartz-regulated escapement) that holds ±0.5 sec/day. Or pure thermo-compensated quartz: Grand Seiko 9F at ±10 sec/year. None of these are needed for daily wear; they are precision flexes by the maker.