Now I know the T-800 in Terminator 2 had a slightly different look. It did look like it had less vertebrae compared to the endoskeleton from Terminator 1. Below are pics of the endo from T2.
And just for the heck of it, here are pics of the endoskeleton from Terminator 3.
Honestly, I think of the endoskeleton from Terminator 2 as the definitive version of the Terminator so that might be why that's what we see/envision when we think of the Terminator.