How many molecules of ATP are actually produced in aerobic respiration?
The discrepancy between 30 and 32 molecules of ATP quoted in the question may well arise from the two possible ways in which the reducing equivalents of the two molecules of NADH generated in glycolysis may be shuttled across the inner mitochondrial membrane into the mitochondrion (suggestion 2). 

If one assumes that the malate-aspartate shuttle is used, then NADH is transported into the mitochondrion as such, and the yield of ATP is the same as for those molecules of NADH generated in the mitochondrion — 2.5 ATP per NADH from the most generally accepted measurement.

However, if one assumes the glycerol phosphate shuttle is used then the reducing equivalents are transferred from NADH to FADH2, from which only 1.5 molecules ATP is produced per molecule. 

In summary, using the malate–aspartate shuttle a total of 5 molecules of ATP (2 x 2.5) will be produced from the two molecules of glycolytic NADH, whereas using the glycerol phosphate shuttle there will be only 3 molecules of ATP (2 x 2.5). This gives a difference of 2 molecules of ATP, which would cover the range in the question.

As the relative proportions of the two shuttles varies between tissues, a range of values is actually possible.