Because when Zuni first introduced highlighting, he made the highlight color green. That is when I updated my signature. Then people complained so he switched the color to yellow. Similarly, all posts and signatures last updated while the color was set to yellow will remain yellow. Then people complained again and Zuni changed the final color to black (reverse). If you post or edit your signature with highlight tags now they will be highlighted in black, even if Zuni changes the color again.
If I were to edit my signature now the highlighting would turn black.
l2c