Typically, barrel length will be measured from the end of the tenon to the muzzle. The bolt face is recessed and has some clearance from the tenon in the Ruger, so it ends up an extra length. A lot of folks measure from the gas port hole in the action, since this should mark the interstice between the bolt head and the barrel tenon. That difference doesn’t typically end up ~3/4”, however.
But…
Is yours a really early model? I remember the early 16.5” barrels being reported at 17.25”, and the early 18” models at 18.7”. I haven’t heard of any contemporary manufacture models with incorrect lengths, but I suppose it was possible before, maybe just as easy to occur now for whatever was the reason it happened before.