There is always potential for human error, yes. But the thing about calipers is that they are measuring skin folds from specific sites. Lets assume the sites were accurately measured. Where the inconsistency arises is in that some areas/regions of the body may be unaccounted for. Lets say a person has a genetic predisposition to accumulate higher amounts of fat tissue in the upper arms. Even a 5/7 point test will miss that. Plus, it is only measuring sub cutaneous fat, which does not account for the interstitial areas. The electrical machines you mentioned are even more inaccurate. They are measuring electrical resistence. Since water is a conductor of electricity, the presences of food or fluids will give false readings. In fact, anything outside of fluid balance will yield a false reading. Water losses occur in many ways, to include sweating and respiration. Hydrostatic weighing will give the most accurate overall reading.

One advantage that skin fold calipers offers over the rest of the methods is to gauge fat distribution and pin point areas where fat has been reduced. For instance, if you measure today and the ab reading was in the mod 20s, then you measured in 6 weeks and found it in the teens, I bet you'd be pretty excited to know that you've lost fat in the ab region. So, this is one aspect that gives this method the upper hand.