Point estimates do not serve a business well when uncertainty is wide, and history shows a range of real price uncertainty that is a factor of two to three times the central figure. Ironically, it seems that most oil companies treat production uncertainty with more sophistication than price uncertainty. Perhaps because inaccurate precision is easier to tolerate than uncertainty.
Purchase and investment decisions rely on cash flow analysis, and cash flow analysis requires assumptions about future commodity prices. You can call it indistinctly an "assumption" or a "deck", but it is still a forecast of what produced oil and gas will sell for in 2018, 2019 and all the way through about 2050.