Yeah, I have seen quite a few different variances of the multipliers throughout different NHRA and IHRA literature as well. But they are just to roughly estimate what your 1/4 ET would be, and if we take them to mean just that they are a pretty good tool.
Only thing is they don't account for alot of the variables like has already been noted. It is not uncommon for a relatively stock car to rip some awesome 60 ft, 330 ft, and 1/8 mile numbers, especially with alot of gear and slicks. The big end of the 1/4 is where the HP really factors in. So if this same guy were to run an actual 1/4 mile, he might actually be more in the upper 12's.
Nevertheless, he has a car that is digging pretty hard for a relatively unmodded chassis/suspension.