They don't have the same specs because they are different fans. The redux fans used to be their Premium line. I bought what is now in grey when they were in beige and brown.I get the sales and marketing aspect of it, but I have to say, that's actually not true. I'm not sure if I misunderstood you or if you misunderstood me, but the fact is that the Redux line of fans DON'T have the same specs as the standard and Chromax lines have. They are a reduced spec series although I do understand that they have a lower price, it's like having a four cylinder corvette.
I bought those Premium fans before I was a reviewer. Noctua sent me pairs of their redux line to review. Because I threw nothing away, I was able to hold the old Premium and new redux fans in my hands to compare them. They were identical, except that the corners were changed to accommodate Chromax corners. I reviewed the NF-P12 on June 27, 2018, at the other Overclockers (I'd send you the link, but we are not allowed to do that). The fifth image shows the Premium P12 and the redux P12 together. Same fan. They differ from the NF A12x25, (the review was published June 20, 2018, at the same site).