Your Panasonic is no better than the Bosch. The difference in range comes from, the type of terrain and how hard you pedal.
50 miles at 15mph on the flat with no wind takes about 800wh of energy. In the real world, you can expect somewhere between 50% and 100% more as an estimate. Let's leave that aside for the moment. That means for a range of 50 miles, you have to provide 400wh of energy along with the battery's 400wh, which is about 150w continuous. That's if your motor is 100% efficient, which it isn't. An optimistic guess at its efficiency would be 65%, so now the motor has only provided 260wh, which means you would provide 540wh or about 170w continuous. That's quite difficult. I suspect that your average spped over a range of 50 miles would actually be a lot less. I find that to average 15mph over any distance requires just over 20wh per mile over mixed terrain with a bit of wind, which would give a 400wh battery a range of 20 miles. I don't believe that the Panasonic motor is more than twice as efficient as mine.
When I average 12mph, the consumption goes down to about 10wh/mile with moderately hard pedalling, which would give your battery a range of 40 miles. Maybe the Panasonic system is more efficient than my Xiongda, so 50 miles might be achievable at that speed.
It's a shame more people don't post actual recorded data about speed, distance and consumption. For that, you need a wattmeter or you need to run the battery completely flat. Unfortunately, it's not easy to use a wattmeter with Bosch, Panasonic, Kalkhoff and such systems to get true data, which is why we get so many myths and discrepencies.
It still comes back to the same thing though: Your battery helps you along. If it can take you a long way, it isn't helping you much, or to put it another way. If the same rider uses two different bikes and finds that he can go further on one than the other with the same battery, he will have more tired legs from the one that can take him further.