Are you sure that you have used the correct amount of water to mix the Dektol?
If you haven't added all the necessary water this time, the colour might be different.
And Dektol packages come in two sizes - any chance you have used the mixing instructions for the smaller size to mix the larger size?
Finally, are you using something like well water or particularly hard water to mix the Dektol? Variations in water quality can have a dramatic effect.