Scientists tend to refer to blood constituents in terms of dry weight when making statements of amount by comparison. By dry weight hemoglobin comprises about 97 to 98% of a red cell. If you include the water measure of a red cell then, by comparison, hemoglobin makes up approximately one-third of the red cell.
Interesting....
So the Society had 2 options when writing about what percentage of a RBC is comprised of hemoglobin.
They could have gone with the "dry weight" method, 97-98%, and most JWs would likely think, "ooh, that's pretty close to almost entirely a RBC, better not accept it."
Instead, they used the "with water" method, emphasizing that just one-third of an RBC is composed of hemoglobin. JWs would then have a tendency to think, "One-third? That's less than half, so I guess that means it is a 'minor fraction'. Maybe I can accept it after all."
If correct, that lends credence to the other posters' ideas that this is a softening of policy.