Most of the copper tubing we see on the general market is either Type K or Type L, and sometimes Type M, and all come in either drawn tubing (hard copper) or annealed tubing (soft copper). The difference is only wall thickness for these common products with Type M being thinnest, Type K thickest for higher pressure service. The other difference that we see is that tubing designated "air conditioning/refrigeration (ACR)" is by actual OD dimension vs nominal OD (1/8" larger than the tube size). All tubing carrying the ASTM designation is 99.9% pure copper (according to the Copper Development Association), and is deoxidized by adding a small amount of phosphorus during processing.
I can't explain why the weld backed by tube vs buss bar material is different, other than perhaps the buss bar being dead flat, and the tube being pounded flat and having a bit of air gap that expands when heated? There's likely a better answer than that, but I don't have time to look any more right now.
Thanks again for the tip!