I have also found that there can be a big difference in feel despite similar stiffness ratings.
There's a couple things I've theorized, but of course have no way to prove without some in depth experiments and complicated tests/machines:
I feel there might be a difference between static stiffness and "dynamic" stiffness. The machine only measures static stiffness, i.e. put the frame on the machine and it slowly flexes the frame and takes a reading. Can we be sure that all rackets that get a reading of RA70 in this test will then flex the same way when a ball strikes the strings? After all, the ball could be traveling as much as 80-90mph even after bouncing after a hard first serve PLUS the returner's racket head speed... the ball will hit the strings/frame pretty hard.
I seem to have experienced this difference in dynamic flex personally with some of my rackets. One great example is that I have a Head PT280, and a Head PT57a. Technically they are the same mold and same racket, but the pt57a is lighter and more flexible as it is a pro stock. When I lightly hold the top of the grip and hit the tip of the head with my other hand, the PT280 resonates slightly higher pitched, indicating that it would be a few points stiffer, but overall they are close.
When I hit with them, they are both soft, but clearly they are different from each other. The PT280 is pillow soft. The PT57a, in contrast, feels springy soft. And even matched, the PT57a also gives me more pop, reinforcing the impression that it is more of a spring while the PT280 is more of pillow that is muted and soft, but also feels like it is soaking up some of the power when you hit. And don't get me started on my Donnay X-Red 94+! The feel of those things are so unique.
As someone else mentioned, WHERE the racket flexes can make a big difference as well. So maybe two rackets are RA70, but one is very stiff in the handle and throat but flexy in the hoop; and the other is very stiff in the hoop but very soft in the throat. When they get a reading of RA70, this is an average, but tells us little more. The 2 rackets I described surely won't feel the same when hitting.