That's why you need David's book. Fiesta Glass was the management buy out of the Fiestaware (the product name) part of Chance Glass when Pilkington's (the then parent company) closed down the Smethwick Chance factory. So yes they are a different company but they did continue producing some, but not all, of the Chance designs, and they did invent some of their own.
If it's floral with a white background or a mix of lace and flowers, it's probably Fiesta Glass, post 1981 and not Chance Glass. If the printing is topside it's definitely neither.