The Shimano Ultegra (6770-A) was an emergency stop-gap model that Shimano introduced around 2015. Shimano had been very happy to clear their stock of the 10-speed Shimano Ultegra (6770) as quickly as they could before it was made it obselete by the arrival of the 11-speed Shimano Ultegra (6870 SS). But then customers who had bought Ultegra (6770)s started to trash their derailleurs and wanted replacements. Shimano had nothing to offer them because they could not manufacture any more because the Ultegra (6770)s used a motor that was no longer made.
A legion of users of the Ultegra (6770) were furious at having bought a very expensive and prestigious derailleur system that was only a few years old but was no longer supported. To pacify these customers Shimano developed the Ultegra (6770-A SS), this used the physical components of the Shimano Ultegra (6870 SS) controlled by new electronics that make the derailleur a 10-speed rather than an 11-speed.