Ken,(posted 8115 days ago)I'll try to keep this short if I can. There are two things going on here. The first is that manufacturers determine their film speeds using very unique controlled exposure and development parameters. Usually the film is developed to a higher contrast index than most of us like to use, and with a developer that is not commonly used. Therefore, one must do personal tests to arrive at the proper film- speed for one's own developer and contrast requirements. This is usually slower since developing to the desired lower contrast usually means less developing time which effectively lowers film speed. The one-stop example that you cite is not uncommon, especially when a long-scale negative is desired.
The second reason to rate film slower than the manufacturer's EI is to intentionally overexpose it. This can be extremely helpful when separation in the shadows is very important and the film being used has a long toe. The overexposure moves the important shadow details up onto the straight (or straighter) line portion of the characteristic curve, thus increasing shadow detail. Of course, this sends the high values higher by the same overexposure. If the film used has a curve that does not shoulder to soon, then there is still adequate separations in the high values. A general increase in grain is the down-side to this practice. For small film users, it may not be worth the extra shadow separation. For large-format photographers the increase in grain may be insignificant.
Hope this answers your question satisfactorily.
Regards, ;^D)