It very well could be bullshit! I have yet to find a better answer tho!
Here's some of the full deal:
All EC/ppm/TDS/CF meters start by measuring the electrical conductivity (EC) of the solution. [This EC value is measured in terms of millisiemen (mS) or microsiemen (µS) per centimeter... 1000 microsiemen equals 1 millisiemen/centimeter.]
Because of this, an EC reading is basically universal (anyone can share an EC value and it will mean the same thing to everyone else). CF is simply EC x 10 [So a 1.0 millisiemen/cm EC equals a CF of 10] and is also universal, just not as commonly used and rather redundant].
Where it gets confusing:
After making an EC reading, TDS/ppm meters will automatically apply a conversion factor to guess at what the parts per million (ppm) is of anything that is dissolved in the solution being measured. Different uses/industries/locations use different conversion factors to guess at the ppm reading. For example, in the United States a .5 conversion factor is most commonly used: an EC reading of 1.0 millisiemen/cm will give you a 500ppm result [Hanna & Milwaukee meters are usually using a .5 conversion factor]. In Europe, a .64 conversion factor is most commonly used: an EC reading of 1.0 millisiemen/cm will give you a 640ppm result [Eutech meters are usually using a .64 conversion factor]. In Australia, a .7 conversion factor is most commonly used: an EC reading of 1 millisiemen/cm will give you a 700ppm result [Truncheon meters usually have both .5 or .7 conversion factors available for use in settings].
Because of this, and the failure of most people to state the conversion factor used when sharing their ppm readings, it's not really a universally accurate way to discuss nutrient solution strength.
The reason for different conversion factors:
Since the only true way of measuring total dissolved solids is impractical (evaporating all the water and weighing what remains), some baseline readings based upon the electrical conductivity of known solutions (and known TDS) were made:
TDS - NaCl: 0.47 to 0.50
TDS - 442: 0.65 to 0.85
TDS - KCl: 0.50 to 0.57
But since in hydroponics, we aren't growing in NaCl/442/KCl solutions... the true ppm is nothing but a close guess. But those measured solutions above are the reason for the range of conversion factors that exist as "close enough" for our purposes.