I agree with Alex. Common standard parallels are used in some regional sets. The publisher in question addresses this in its
FAQ:
Q. Why does the scale of the VFR+GPS charts differ slightly from 1:500.000?
A. In order to support the European wide standard, a Lambert Conformal Conic Projection with Standard Parallels at N37° and N65° is used for production of all VFR+GPS charts. This enables the seamless combination of all charts and the use for continuous flight planning. A scale distortion is a consequence of the projection and varies with the distance from the Standard Parallels. The true scale is depicted on every VFR+GPS chart on the Cover panel (e.g. EB/EH: True Scale at N51° - 1:515.000) and near the Scale Bar. In addition, a factor is given to multiply measured distances with every parallel (e.g. 1.03).
greater accuracy can be achieved by selecting two SPs at a range closer to 16 [deg]
While this may be true for the chart in question it obviously depends on the actual latitude range to be charted.
I also must admit the math level ... is beyond my current knowledge.
Ignore it. The pertinent information was in the text and was adequately summarised in Deetz and Adams.
In practice because the scale on these projections is almost constant it is easier to assess distances by referring to a meridian graduated in minutes of latitude.