...there are several reasons for setting back a barrel.. like Vapodog says, there could have been a situation of excessive headspace, which can be successfully corrected by setting it back and rechambering. Since this is a varmint barrel, it could also have been shot enough to erode the throat, degrading accuracy. Setting the barrel back and rechambering/throating can fix that problem, also. It could also be that the owner wanted a "match" chamber, which has tighter dimensions than SAAMI standard specs. If it was caused by excessive headspace, I would have the bolt lugs checked for setback/cracking. A competent gunsmith should be able to confirm the rifle's "shootability" and safety.

"Progress, far from consisting in change, depends on retentiveness..those who cannot remember the past are condemned to repeat it." Santayana