Correct, as they are really 2 different tubes with 2 different design specs. The original GE 6550 tubes were in many cases overbuilt, but not for audio reasons. The 6550 was a common servo amp tube that was used in high voltage servomechanisms. This means that they might be subject to rectified high line voltage (3 phase supply) in order to drive high power AC and DC motors. Since they were most often specified as industrial and military application tubes (driving things like big data recorder reel motors, up to big servos for gun positioning on battleships, rudder control, etc.), it was not uncommon for them to be overbuilt. Audio is pretty much an afterthought for these tubes, and since they are no longer used for their original intended applications, there was no real market driven need to overbuild and overtest compared to the "good 'ol days".
One way to understand why some tubes might be built to a particular standard is to look back at their originally intended applications. These will usually give you a hint as to WHY. For example, the 6L6 was used for servos as well, but was also designed as an AF power tube as was the 6CA7. These were common is the cinema tube amps of the 50's and 60's, there were lots of these amps AND the tubes logged a lot of operating hours weekly, AND money was not a big object to the cinema industry (back then) so tubes were routinely tested and replaced and it was not a big deal. Since the application demands were not as rigerous, they were less expensive which is what drove the early guitar amp manufacturers (in the US) to use the 6L6. Besides, it was a good mix of sound quality, mechanical reliability, electrical reliability and cost. Same applies to Europe and the EL34
