Rounding from 3.8 to 3.9 is insane. I work in the Aerospace industries where close tollerances are critical. But I also have seen when parts are oversized or have a particular dimensional problem they will evaluate the problem and if the run of parts made wrong cost to scrap may exceed desired levels and will put them in production with a update in the tech manuals and tech references as well as all maintenance manuals. Hel knowing Ford they probably had 10,000 pistons that were manufactured to close tolleranced to use in the 3.8 so this was there solution.
It is all about profit