Why are tweeters so high off of the ground in many tower speakers...

...when most peoples ears are much lower when seated??

I've read many times that the tweeter should be at approximately ear level.  Well, I am 6 feet tall, and sitting straight up on my couch my ears are about 40 inches off the floor, lower of course if slouch.  My Artemis - Eos speakers have the tweeters at 45 inches high, so not too bad, but many others that I've seen are 50 inches high and more and I don't understand the designer's thinking.   Is a 10 - 12 inch difference in height acceptable or should the speaker be tilted down, which I think would create a whole new set of issues.