I believe this is the same as saying that Christ redeemed us from the curse having become a curse for us, Christ shared in our infirmity. The Father sent the Son, but at the same time the Son set aside His own glory to become as one of us.
The description of "sin offering" is simply looking at the aspect of the Father sending or offering the Son - or the Son laying down His own life - to redeem us from our sins.
I think that this also is related to Christ set forth as a Propitiation (as a "propitiation" is a "sin offering" that turns aside wrath; it is a sin offering with wrath in mind).
If it helps clarify things,
@Martin Marprelate , I do not think that using the term "sin offering" is what separates our view. I know that you would agree that God set forth Christ as a Propitiation, and that Christ lay down His life for our sins, etc.
I think the difference does have to do with what was accomplished at the cross (we both agree it was redemption, but how that redemption was accomplished).