Melanocytes are specialized cells in your skin that produce melanin, a dark pigment.  Melanin protects your body from sun damage, by stopping harmful sunlight from reaching the lower layers of your skin.  Everybody has melanocytes.  Melanocytes make more melanin  in response to more light, and less melanin in response to less light. 

If you go out into the sun, you clothing  shelters your skin from the sun.  You will have lighter skin where your skin was covered, and darker skin where the sunlight reached the melanocytes.  The melanocytes made more melanin in response to the sunlight on the skin.  Henna stained skin cells also block sunlight.  The henna stains are in the stratum corneum layer, which is above the melanocytes.  Therefore, if your skin is hennaed, the melanocytes will not respond to the sun, and will make less melanin.

If there is a henna pattern on the kin, the henna stained cells will block the sun in that exact pattern..

As long as the henna stain is on the skin, the melanocytes under the pattern will be sheltered and make less melanin.  The melanocytes still exposed to the sun will produce melanin to shelter the skin, and that skin will appear darker because of the increased melanin.

When the hennaed cells have exited the skin through exfoliation, the skin cells that appear suntanned will be at the surface of the skin and will appear as a "reverse henna:"!

The melanocytes may be stimulated to produce extra melanin for several months, so the reverse pattern will disappear very gradually.