Bending is a result of moments and moments are driven by moment arms. These tests should not be looking at point load required to induce deformation but rather the moment required to induce deformation. Each of these should have been converted into equivalent moments based on the size of the phone.
For example, the M8 is shown as 146.3 mm (5.76 in) H; 70.6 mm (2.78 in) W; 9.4 mm (0.37 in) D and the iPhone 6 is 138.1 x 67 x 6.9 mm (5.44 x 2.64 x 0.27 in) and both are shown to "deform" at 70 lbs.
The M8 has an induced moment of 100 lb-in while the iPhone 6 has an induced moment of 95 lb-in. The resultant bending stress by assuming a linear stress distribution across the section is then 1575 psi for the M8 and 2950 psi for the iPhone 6.
Similar analyses should be undertaken for each of the other phones.
A more accurate test for the failure mode of concern would be a four point load test in which moment is constant across a portion of the phone. The three point load test induces a moment that is maximum at the point of load application. The four point load test is more likely to show you where the point of weakness in the phone is.