This has had me very curious for a very long time, one might even say it has always drove me nuts. lol

(all though i'm struggling trying to put it into words)

one example, would be a fan prop i have modeled. centerpoint of the prop is where the origin is located. (see image)

when looking at the front view (same as image) using the Alt+arrow to rotate the model along the z-axis, it stays centered for a various number of incraments, then it moves.

one would think, if i was looking at the front view and continuously spun the model by holding the Alt+arrow key, that it would spin as if a motor was spinning the prop. but if i do so, the model only breifly spins about the z-axis, then it starts spinning all crazy.

does anybody have any clue what i'm talking about? lol

again, this is not creating problems, just trying to understand why it does what it does.

EDIT:  I have just discovered that if i zoom out (no specific distance) it spins along the z-axis perfectly for eternity, but as soon as i "zoom extents" it acts as if i selected a face and goes back to being crazy...lol