Monetary value is based on what a knowledgeable buyer is willing to pay for it. On a sliding scale, rarity typically sells for more. If (in some alternate universe) original M1 carbines and Garands were more common and rebuilds were rare, then rebuilds would probably be worth more to collectors (with just as many folks faking them).
Keep in mind too that the values in the various collector books are set by collectors of those particular weapons. Folks that collect every variation of Garand, or K98, or Colt SAA, are going to be a lot more picky about the historical details of these arms. They want to differentiate the many variations in their collections.
On the flipside, there are a lot of less discriminating folks who just want a representative weapon of a type or various types. They may want "a Garand", "a K98", or "an Enfield" for their collection. They aren't going to be as picky about when it was made, if all the parts are 'correct'. They just want a reprasentitive of that type. Overall condition would be more important than technical and historical details. If they are looking a details, they are probably looking for just one or two things. If it's a type 99, does it have a mum? If it's a Garand or Carbine, is it a WWII rifle or a later Korean war rifle? If it's a K98, does it have swastikas? If it's an Enfield is it British or Indian? And etc.
I mostly fit in the latter category, and I suspect the OP does too.