I believe the difference in the label of 15% available chlorine and 12.5% sodium hypochlorite may be difference in units - weight vs concentration.
The difference in test results has to do with freshness. Bleach (sodium hypochlorite solution) degrades very quickly when exposed to warm temperatures and sunlight. The manufacturer / packager bottles it at a higher concentration than the label in the hopes of still having the label concentration when it gets to the store.