A current trend in biofilm research uses viscoelasticity to evaluate biological mechanisms used to adapt to environmental changes. However, due to un-quantified variability in mechanical properties, it is difficult to make comparisons between different growth conditions, strains, or species. To interpret biofilm viscoelasticity requires understanding its statistical variance divorced from experimental technique. In this work, 23 P. aeruginosa biofilms at air/water interfaces are characterized using an interfacial rheometer. This provides the least disturbance to the biofilm, allowing a truer sense of biological variability. Experiments were conducted in 3 sets in 2 different labs. Moduli varied 1 order of magnitude within a single set. Statistical analysis confirms that under seemingly identical growth conditions, viscoelastic moduli were different based on experiment location, which is attributed to minor changes in ambient conditions. Understanding this variance will lead to a better evaluation and comparison of biofilm formation in future studies.