My standard pack has been an older (2007 or so) model of the Gregory Z55 with some stuff removed. I haven't liked it very well for a lot of reasons (mostly the pressure on my neck), but I haven't had the money or inclination to replace it until now. I've also hiked with a GoLite Jam a little bit, and I get the same problem. I've tried on other packs which seem to "pull" on my neck to varying degrees. The Granite Gear Vapor Trail was the worst, surprisingly, and the Osprey Exos seemed to do it the least and hug my back the closest, if that tells you anything.
Interestingly, my first real backpacking experience was with NOLS about 5 years ago using a 7lb+ monster pack and 40-75lb loads, and although it was painful in a lot of ways, I never really had that sharp pain in my neck.
By "fitment", I assume you mean pack construction relative to my body. I don't really know what the relationship is between different pack elements (load lifters, strap width, etc.) is to this problem I seem to have. But I suspect there is a "fitness" element too, in the sense that I have a slight frame but a really long torso, which might cause my back to buckle a little under heavy weights and put more pressure on my neck. I'll need to beef up and work on those upper back and shoulder muscles.