My bike also came from the factory as a hybridized mtb. According to the '97 Fisher catalog:

s.u.b. A.K.A. Sport Utility Bike. The heart of these bikes are mountain bike frames. Because of light, fast-accelerating 26" wheels with slick tread tires and bigger gearing, these bikes go like hell on pavement. And they're no slouches when you stray off the beaten path. An S.U. B. is stronger, faster, and lighter than some generic mutation hybrid bike: they're brand new and they really work.

Interestingly enough, the frame of the '97 Nirvana isn't identical to any of the '97 Fisher mountain bikes. It has slightly steeper head tube and seat tube angles, and a lower bottom bracket, besides other differences. I like the fact that it has more road bike-ish gearing. It's been a great do-anything type of bike.