Models: Why does cross fitting restorations made on the SLA fail to fit on stone models?

My doctor and I are new to digital impressioning. We were both a little nervous so he took a traditional impression for the first few cases. I built up the case on the blue SLA model but then I checked it on the stone model and it didn't fit. I'm very concerned that this case is going to fail. How can the restoration fit on the SLA and not fit on the stone model?

With any manufacturing process, there is some variation. If you pour plaster and manufacture SLA in parallel, both models will vary slightly from the object that they are replicating. Some of this variation between the two objects will overlap, hence crowns made on SLA will likely not fit on plaster and vice versa. Both however will likely fit in the mouth, as both the plaster and the SLA are biased positive relative to the mouth. In stone, this bias comes from stone expansion and in SLA, we apply the bias digitally.