It usually means they are spot metering and have used the sky to point it from and calculate the exposure time. Metering off the Sky oppoisute the sun is usually a safe method for getting an even exposure.
Matrix metering and applying some exposuure compensation is the easiest option I think for your sitiation. If you have a spot meter available though, try it - point at object A and look at the hutter speed. Then point at object B and see how it changes...