The long term fuel trim is comparing the actual fuel trim (fuel pressure x duration of the pulse) to a table of theoretical values stored on a table in the computer that was derived from the engineers at the manufacturer. The actual fuel delivery during closed loop (engine warm) is determined by the O2 sensors, MAF, and other sensors.
Your air/fuel ratio is still ideal and the engine is not running lean or rich. As the engine ages, this LTFT will change as the sensors and the engine ages. It may even slowly change to near zero and then go positive or it may continue to go further negative. When it gets to about 20% either way, you will get a check engine light. Most likely at that time, the MAF sensor will be dirty, but it could be a marginal O2 sensor too.