Dave, it's likely you have not only a luan wood roof substrate below the rubberized roof cover, but also a layer of 'styrofoam' rigid roof insulation layer below the luan that provides for some roof movement, temperature changes, and gives the ability to more easily walk on the roof without the luan being stressed as much between aluminum roof joists, which is why your length of screw may not have quite reached the joist.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada - Northwest, GA! (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )