A typical oil study includes a constant-composition expansion, a differential liberation, a separator test, and viscosity. The tables of formation volume factor, solution gas-oil ratio, and viscosity used in reservoir calculations come from it. The bubble-point pressure is read from the same experiment.
The sample has to represent the reservoir fluid. Separator oil and gas recombined in the wrong ratio, or a sample taken after the reservoir has fallen below the bubble point, produces tables that are the wrong fluid. Every volume calculated from those tables is then wrong in the same direction.