It's due to the fact that we're still living in sinful bodies. It's not that God's grace replaces our faulty will power and makes us incapable of sin, it's that even though our bodies are still subject to sin, God's grace covers it all.
And yes, I would say that should the author sin in the future, that sin will be forgiven - I would say it being dependent on the inclination of his heart towards God. (There's the verse that says "If we say we have fellowship with God yet walk in the darkness, we lie and do not practice the truth." - that's where confession and repentance/turning away from one's sins comes into play.)
Paul's letter to the Romans speaks volumes more on God's grace versus the sinful nature of our bodies than I could ever explain - but it does occur to me that if we posit that all sickness and death comes down from Adam's sin - and that even though we accept God's grace through faith in Jesus' sacrifice on the cross - then it would stand to reason that were it that our salvation prevents us from ever sinning in the future, maybe it could be argued that we shouldn't ever get sick or die either, which is obviously not the case.
I sure do appreciate your honest comments and questions. These are the sorts of things that people do wonder about - as I have done and continue to do as well.
Thanks for the replies!