1990 was the first year of the ZR1 so it still had the old C4 design. 1991 was the first year of the C4's refresh, hence the style changes. The square taillights were supposed to be unique to the ZR1 but GM went GM and included them in the refresh for all C4s.
Also, the VIN on the window sticker would certainly match the one on the dash, otherwise it wouldn't be included in the ad.
Speaking of the ZR1