As has been mentioned, inadequate power cables are introducing significant voltage drops, and as current flow increases, so does the voltage drop across the cable. Then, there's the internal resistance of the power supply itself, which is a demonstration of the Heisenberg Uncertainty Principle - in measuring and especially producing the output voltage, it is affected.
The maximum specs quoted for such low-cost/margin devices as cell phone chargers (made by the lowest bidders on contracts for millions of units, each) can be off by as much as 20% of their rated values because of the non-critical nature of the cell phone charging application and minimal attention paid to quality control. Since such supplies are designed to maintain a constant voltage (a voltage source, rather than a current source), the current can (and must) vary in order to maintain the rated voltage.
However, if a particular supply happens to be on the low side of the performance bell curve for all of the devices manufactured in a lot, it won't be providing enough current, low by as much as ~140 ma for a nominally-rated 700 ma device! That's not even counting the voltage drop through the power cable, either, so, you can wind up with both low voltage and low current from a marginal cable and power supply operating at its full actual capacity (not even what it's supposed to be providing). Sometimes, you don't even get that for which you're paying.
The difference between devices operating at the upper and lower ends of the bell curve isn't a matter of design, it's a matter of statistical deviation in production output, which is impossible to predict for any single device. At the extreme upper limit of this phenomenon are high-power microwave devices, such as mechanical resonance cavities used in microwave production and amplification components (e.g., klystrons and magnetrons) manufactured for radar and medical radiological systems (e.g., for X-rays and CAT scans). One high-end cavity can cost upwards of a million dollars, not because there is that much invested in material and labor costs in that individual part, but, because, in order to produce one specification-compliant cavity, over 10,000 parts have to be produced, and about 9,999 of them don't meet the spec and the materials are recycled.
It's physically impossible to intentionally manufacture such a part - you have to make all ~10,000 parts and test each of them to find the one that is the Goldilocks part - not too big, not too small, but, juuuust riiiiight. The same is true of semiconductor components - the fastest and slowest components in a production run of microprocessors, memories, etc., are all manufactured together on the same lines, and it's simply a matter of testing that determines which ones can fetch the highest prices and the rest will earn somewhat less to much less (and complete failures earn nothing).
I know this may be boring to the hardware folks, but, few others are aware of what it takes to produce every great wonder from a single transistor to an entire Pi.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!