Lenny,

To be sure, the two-dimensional design of the system is not without certain limitations -- one being they will not always point you in the exact direction of your destination. Also, remember the system is designed primarily for Portland visitors and to that end directs pedestrians to the most-walkable -- not necessarily the quickest -- routes toward their destination. My hunch is that in your case, the sign telling you the main library was to the east was pointing you toward another sign a block or two down that would direct you south again down a more-navigable route.