Nofool, I agree with part of your explanation.
I agree that the offer is for the purchase of 15M shares; this represents 15.9% of the outstanding shares. If all shares would have been tendered, then the full 94.3M shares would have been deposited and only 15M would have been bought back, the buy back would have been at 15.9% for each request.
In our case, 42.7 M shares were tendered. Consequently, individual requests were filled at 35.06% (15M / 42.7 M = 35.06%). This means that if their full 17.1M shares was tendered, the buy back would end up at: 17.1 M x 35.06% = 5.995 M shares. Since their buy back is at 2.8M shares, it meant that they have not fully tendered all of their shares, i.e., 7,993,730 to be precise: 7,993,730 x 35.09% = 2,805,000 shares.