Why is there a difference in price (poor correlation) between Gold Petal (1gm) and Gold Guinea (8gm)?

22 August - Guinea Close Price 22616; Petal Close Price 2941

25 August - Guinea Close Price 22482; Petal Close Price 2814

Since Guinea is Gold 8gm and Petal is 1gm, by theory, Petal price has to be 1/8th Guinea price . So on 22/08, Petal = 22616/8 = 2827. But the market close price of Petal on that day, as shown above, is Rs. 114 higher. Why is there a big difference?

The ratio between Gold Guinea and Gold Petal is 1:8

But there is some deviation due to other market forces and factors such as volumes, demands on that particular contract.

So, we can't say that the price moves accurately in 1:8 ratio every time. 

I found the answer myself. Gold Petal is 99.9% pure and Guinea is 99.5% pure. Hence Petal prices per 1 gm are always than Guinea prices.