Fantastic Concept, Kelly.
I tend to agree with Dale, I think it is in the difference between your saw Kerf, and the thickness of your metal insert.
Can you measure across your saw blade at the cutting edge that is the maximum tooth offset, then measure the thickness of your Insert.
I expect that this difference is main cause of the slight offset you are experiencing.
Also it will increase the Total difference with each additional subsequent cut made.
Please let us know of your measurements?
Great work,
Brian.