1999 Chevrolet Blazer odometer problem

1999 Chevrolet Blazer v-6 Vortec mileage: 12,000 We were driving on the interstate last week after an ice storm and had 4-wheel drive engaged. We hit a patch of ice as we were accelerating to avoid a motorist who lost control and all wheels slipped and spun causing the speedometer to "bounce." After all the excitement the gauges were "bouncing" and we noticed the odometer had approximately 20,000 more miles on it than it had. Why would this have happened? Isn't the odometer tied into the revolutions of the drive shaft? Our dealer said it was impossible and we even had an invoice where we had our Blazer serviced a week prior but with 20,000 less miles.