Sometimes it takes years to define the perimeters of something; some crucial detail eludes me. Here goes: Tests of flexibility are generally done with the legs straight/ extended. It appears that, for me, though, the ability to bend at the waist depends, to an unusual degree, on also having bent legs. In P.E., students are asked to sit with their legs extended and then touch their toes. The act of sitting with my legs extended is in itself uncomfortable; my range of motion in this position is pretty limited. But if I bend my legs deeply -- sitting on my feet, say, or sitting in yogi position -- then everything changes. I can not only bend my body flat against the ground, but I'm so comfortable I could sleep that way. (Sometimes I do.)
Pictures taken here and there, across my life, are suggestive of loose joints. If you look at the picture in With Legs Crossed, you'll see a small child with some unusual posture. In that pose, it almost looks like my legs detached. Either that or they were pretty durn short... which they weren't. I doubt I could do a toe touch, though, even then. By the time I started school, at any rate, that task was way out of reach.
Once again, I've got to get on my soapbox and say that it's not just about physical capacity. If we look at how a person acts in one context, striking as it may be, we can get into some trouble generalizing or making predictions about how they'll perform in other contexts.