Why my long fuel trim readings are higher than short trim ones?

You are right the STFT will replace the LTFT after a given amount of time. IIRC, the LTFT is the adjustment to the tune itself to compensate for fueling issues over a longer period of time. If the STFT is large for a long period of time (whether +/-), the LTFT will be adjusted accordingly. If the LTFT is at +17% and the STFT is steadily at -3%, then after a period of time the LTFT will drop to +14%, which should then move the STFT to zero. Then if the STFT needs to move to -3% again to adjust, after a period of time the LTFT would be moved to +11%. I don't know what the "time period" is for when it makes adjustments. I'm sure it's probably different per manufacturer's specifications.

It sounds like everything is doing its job, however, +10% is quite a bit of adjustment for LTFT. I believe 17% is pretty close to maxed out. I'd ensure the O2's are in good working order and the fuel system is clean (including injectors & fuel filter if equipped).