Do they check headlight alignment at service time though? I don't think they do.
Sometimes small things can make them go from being just within spec to just out of spec. Eg, a friend of mine replaced some rear suspension parts on his BMW, causing the rear ride height to raise slightly - the headlights then dipped lower as the car was pointing slightly nose down.
I guess new rear tyres, a slight difference in ride height (air matic?) etc could cause similar.
At the end of the day, garages have better ways of making money rather than charging just £10.00 for adjusting headlights