what is my problem?

Here are my three separate swatches I have made for the cabled hoodie. The one on the orange holder was my first one that I did on size 8 needles before I case on the whole back before Seattle. The one to the right is on size 9 needles and produced the exact same gauge (5 stitches per inch) as the size 8 needles ... what? how is that possible? The bottom swatch is on 10.5 needles and I am getting 9.5 stitches per two inches, which is about 4.75 per inch ... not much better! what the heck is wrong with me? I did the math with my gauge from the 10.5 needles and think it will work out, but I wasn't really planning on using those needles for this project, since I need those needles to finish a scarf I am half-way done with and to make more felted bags. I don't really want to give up those needles to a long-term project like my cabled hoodie. It looks like I'll have to make another run to AFY at lunch to pick up some 10.5 circulars (which I prefer to use for larger pieces like the backs of sweaters anyway) and do one more gauge swatch with the new needles before casting on. At this point all my yarn is going to be gone before I even get the right-sized sweater started!