GSkill seems to go through spurts of new models with problems more so then other suppliers.
Most(if not all) of their ram meets a JDEC spec of 1333, higher speeds are Intel XMP or AMD BE settings to OC the sticks, true JDEC spec 1600 speed or faster is hard to find and expensive when you do.
Personally I think GSkill is trying to push the sticks too far on the lower voltage spec, raising the voltage above the 1.5v published recommendation is not advisable.