-23 means the ignition is 23 degrees advanced at idle, but only occasionally because it is at 0 most of the time. thats not a problem, you would expect the ignition to advance when the ecu determines that ignition advance is needed to make sure all the fuel is burnt in time. it could also retard slightly depending on the conditions, but overall at idle you would expect to see 0 more often than anything else. if it was constantly at -23 i would possibly think overfueling was happening, and the advanced ignition was used to compensate for that, but on modern cars that kind of situation usually only happens if a sensor fails that affects the fuel injection (so ignition is used to compensate that lack of control on fueling), where as on older cars the carb set up would be suspected.
if you can repeat this test you should try accelerating the engine speed, this should show an increase in the ignition advance. this happens to make sure the air-fuel mix has enough time to burn at higher rpm. at higher rpm there will be more fuel and less time to burn it, so ignition advance is needed. if it was not advancing correctly you would notice a loss of power and high emissions (unburnt fuel).
lets assume you did notice a lack of ignition advance at higher engine speeds. traditionally advance was controlled by vacuum. although your car (all modern cars) now use electronic ignition systems, there is still an electronic method used to assess the vacuum. the EGR pressure transducer on your car sends a signal to the ecu, which lets it know the vacuum condition. if this was faulty the ecu would not recieve this information, which could cause the ignition advance to be incorrect (retarded in relation to engine speed). this isnt noticable at idle, but under load you would see power loss and high emissions.
if you have disconnected your EGR vacuum hose then obviously the EGR pressure transducer will be telling the ecu that there is no vacuum, which would retard your ignition.
hope this helps. i really should charge for this type of info, garages dont give it out so freely.