well, i would guess because your other examples have noticeable characteristics of the breeds involved.
however, great danes do not have that.
greyhounds and mastiffs have floppy ears. great danes often have more pointy and slightly more upright ears--not common to either of the 2 original breeds.
greyhounds and low and skinny, mastiffs are rather stocky, yet considered large dogs.
great danes are taller than either breed originally, and is a of slightly more appealing proportions than the two (not too skinny and wiry, not to stocky either.)
greyhounds have long, skinny, pointy noses. mastiffs have short, flat noses (somewhat like a bulldog, only not so prominent a lower jaw).
great danes have quite square noses, both from a front and side view. and not as droopy jaws than a mastiff. not hardly.
honestly, i am wondering where you found that fact of the great dane mixture, because it doesn't seem as if that can be true, based on the looks. i mean maybe a little bit in common, but it really just doesn't click in my mind.
however, if that is indeed a fact, then i guess i learned something new today, didn't i? :)
why not try mixing the two and let us know what you got? ;)
http://cache.eb.com/eb/image?id=28324&rendTypeId=4
http://www.elitehavanese.com/mastiff/mastiff-joey.jpg
http://vonknopfdanes.com/myPictures/4.jpg