This appears to be the case for all units - I'm fighting in a province now, and all damage ranges have the same minimum and maximum, for my troops and the enemy.
Thanks all - it is indeed the case that for all units the minimum and maximum damage figures shown are identical, whether in the training camp/barracks, or on the battlefield, though the figures do vary between battlefield and military building. I for example see 6448-6448 on the battlefield for Sword Dancers, but 7383-7383 in the Barracks.