Kidner commenting on this verse…

Psalm 103:3 who forgives all your iniquity, who heals all your diseases,

For all the similarity of these two phrases, there is a difference between God’s handling of iniquity and of diseases, which was made plain in David’s own case when he repented of his sin with Bathsheba. Forgiveness was immediate; but healing was denied, in spite of seven days of prayer and fasting (2 Sam. I2:I3-23). ***If relationship with God is paramount, this makes good sense, for sin destroys it, while suffering may deepen it (Heb. 5:8; I2:11). Yet awe wait for … the redemption of our bodies’ (Rom. 8:23), and enjoy already many foretastes of it.