The theory is that 1949, being the first year of production and with only a limited number of rifles being built, resulted in all rifles going into service and being used extensively. The stock of 1949 rifles were nearly, if not completely, used and abused, and went through the refurb process as a result. After a couple years of production, there were more rifles to go around and more continually being produced, which allowed many later date rifles to be tucked away or kept in a better state of preservation. From my own collecting experience, by 1951 there was ample supply and production of rifles, and there were certainly a decent amount that either went unissued, or were maintained in a high state of preservation.
Added to that I have seen some with more than one refurb mark indicating more than one refurb for quite a few. These were the heaviest used so thus it would stand to reason that because of their relatively few numbers and heavy use it would be very unlikely that any escaped refurbishment. Maybe they are out there but not all refurb have the refurb stamp on them.