da qui
Given the special requirements of this mission, the azimuth required for the alignment of the inertial units was 70 degrees instead of 90 degrees, as is most often the case for missions to geostationary transfer orbit. This gap led to the 20-degree shift to the south in the launcher trajectory from the initial seconds of flight. The cause of the trajectory deviation, therefore, was due to a bad specification of one of the launcher mission parameters that was not detected during the standard quality checks carried out during the Ariane 5 launches’ preparation chain
io intuisco (più che capisco) che la macchina avesse il piano di volo impostato per un azimut di 70° a non le unità inerziali che erano allineate al resto della macchina come al solito (90°), quindi nei primi momenti di lancio qualche sistema di controllo leggendole ha azzerato l’errore portando il volo a -20°.