explained that it all revolves around changes SAAB made to service intervals from 2 years/18000 miles between services to 1 year/18000 miles.
What she has told you isn't quite right, the service interval for the old 9-5 (don't ask me about the "new" 9-5 as I don't know) has always been yearly. The pre-2004 9-5's were to be serviced every 12k or every year, whichever was soonest but from model year 2004 onwards the interval was raised to 18k or every year, agan whichever was soonest. The 9-5 was never on a 2 year service schedule. The 9-3 had a 2 year or 18k schedule which was also changed in 2005 (I think, can't find the exactly year now) to be 18k or 1 year, whichever was soonest. There was also an exception to this, the 3.0 diesel always had a service interval of every 12k.
I've always suggested that people still got an annual service with those earlier 9-3's even though Saab said it wasn't necessary as by 2 years the oil would be very black no matter what the mileage was and it was very often at service time that issues like worn bushes or heavily worn brakes were spotted well in advance before they cut tyres out or damaged brake discs etc.
Of course, there's another caveat in all this, if you're not doing the mileage to reach the main service intervals then you can have what's called an "annual service" which is basically an oil filter and check over of the car, some people on low mileages would end up having an annual service every other year.
Clear as mud I think?