If we are to assume that the labeling is correct, (a safe assumption) then the manner in which you calculated the vintages is roughly correct. It may be that you are out by a year in your calculations as if the spirits were bottled earlier in the year than they were distilled. (Let's say for arguments sake, the first was distilled in September, 1991; but bottled in January 2007, then the age would be 15 years rather than 16, because the spirit did not remain in the barrel the full 16 years. This would be the safest assumption is do the calculation the way you did and then subtract one year to determine the legal age of the bottle.
The Select Reserve contains no age statement. This is a blended single malt which uses malts of different ages (and from different barrel types) to create a signature flavour profile. In the case of the Reserve it is the flavour profile that the distillery is concerned with rather than the age. In order to create the flavour profile consistently the distillery may use an 8 year old whisky as the youngest in the mix one time, but 7 year old whisky the next. From a marketing standpoint, having the age statement on the signature malt of the distillery change from time to time is not an attractive proposal. So the Glenrothes Select Reserve is bottled with no age statement.
If you are interested in my take on the Select Reserve, you can read my review here: Whisky Review: Glenrothes Select Reserve