I had the same question when my midwife said the head was well down. Her response was that everyone has such a different interpretation of the meaning of the word "engaged" that she doesn't use it anymore. She said as long as the head was down, then all is looking good.

Louisa had her head on my cervix for a couple of weeks before I went into labour. You'll start getting those lovely electric shocks into your nether-regions soon as your cervix thins. I honestly thought I had pinched a nerve they were so intense.

I know that doesn't answer your question, but at least you can relax and enjoy the fact your bubba is so clever at positoning itself!!!