yeah cost. and the sticker.

4130 is pretty generic stuff. it's composition can vary within a certain range. and standards say that it is "weaker" than 525.
I'd assume that Reynolds might have slightly tighter tolerances on their composition.

and yeah, as long as 1 tube is 520 then they can put the 520 sticker on.
This is actually quite common, even dating back to the 531 days.
Reynolds made a whole series of 531 tubing decals to indicate which frame tubes were actually 531 and not hi-ten. There were also 531 decals to indicate SG vs. butted 531.