Field of view and true magnification of Canon binos

Here are my measurements.

Canon 18x50 IS:

• The apparent field of the bino is 62 degres (from Polaris to over Alpha And).
• The real field is 3.85 degres, measured to be midrange from Delta Cas to Gamma Cas (3.60 degres) and Delta Cas to SAO 11444 (4.13 deg).
• Thus the real magnification is 62/3.85 = 16x, not 18x !

Canon 10x30 IS:

• The apparent field is 48 degres (from Polaris to M31).
• The real field is 6 degres (from Beta And to 2/3 between Mu And to Nu And).
• Thus the real magnification of the 10x30 IS binos is 48/6 = 8x, not 10x !

I wonder if this is standard practice among bino manufacturers ?