I have had a couple of times on a bike where my body was perfectly warm, but my hands were freezing. My daughter experiences the same thing, and she is much younger and more athletic than I am.

We thought maybe it was gloves lacking wind resistance. We recently bought her several pairs of gloves, it was 10 degrees outside, and we went out on the back porch with a carpet dryer squirrel cage fan (very high velocity and high volume of air output) and tested each pair.

Even with the best pair of wind resistant gloves, her hands still get cold (but definitely not as cold as with crappy gloves). The carpet dryer fan test was great for finding the best pair of gloves. The differences in gloves was immediately obvious. All the gloves said they were windproof, but there were highly varying rates of falsehood to those claims depending on the brand/model of glove.

It's not low core temperature that makes your hands cold on a bike. I don't know what makes them cold. Could be that your wrists are bent back as they contact the handlebars and that compromises circulation. Maybe with street bike handlebars (that curve down) you might not see the problem. But with mountain bike horizontal handlebars, I certainly do.