Brian, I agree that the bc may be off in the bullets what I have a hard time believing is the bc is higher than they advertise. it would be my guess that if a bullet manufacturer was to fib a little it would be to the good and not the other way around.
A couple comments here.
1) Sierra test fires their bullets for BC. Of all the companies bullets' I've measured, Sierra, on average, has the least amount of error in their BC's. Some of their BC's are higher than mine, some are lower, as much as +/- 5% is normal, on rare occasions there can be up to 10% or more error between our measurements. In this case of the 6.5mm 140 grain MK, the BC would only have to be in error by about -4% or -5% to see the discrepancy in path that you observe at 1125 yards.
2) Advertised BC's are corrected for standard atmospheric conditions. The problem is, there are a couple definitions of 'standard atmosphere'. Sierra's advertised BC's are corrected to the older Army Standard Metro (ASM) atmosphere model. JBM uses a more modern atmosphere model; the International Civil Aviation Organization (ICAO). When you use a BC that's corrected for one standard in a program that uses a different standard, you get some error. In this case (using a Sierra BC in JBM), you induce about a -2% error in the effective BC. In other words, JBM is interpreting Sierra's BCs as being 2% lower than Sierra intends for them to be represented.
3) When Sierra tests a new bullet for BC, the BC they advertise hardly ever changes. They'll advertise the same BC that they measured for a bullet 2 decades ago. They may be on their 100th set of bullet dies and the bullet's shape may be quite different than the original samples that were tested, but I've never known Sierra to re-test and update their advertised BC's. This is another possible reason why their advertised BC's may 'drift' a little from actual values.
The point is, I wouldn't necessarily assume that when a companies advertised BC's are off that they're always high; sometimes the error is that they're low. Also, the error isn't always intentional.
Measuring BC's isn't very hard.
Measuring BC's accurately
is very hard and requires extreme attention to detail.