It's Ruger, it's not the gripmakers.
Ruger's manufacturing process results in varying gripframe dimensions.
Sometimes you'll get a pretty good fit, other times you'll get an OK fit, and occasionally you'll get a lousy fit.
The gripmakers like Altamont fabricate to a fairly consistent standard.
Ruger does not do the same with their gripframes.
And Ruger does not fit grips to frames. Period.
The worst I've seen have been on stainless single-actions.
The older alloy gripframes tended to be more consistent, since they came out of molds & were not machined before being assembled.
There's something in the steel gripframe process that causes dimensional variances.
Those are machined & fitted to main frames.
If you want a perfect fit to help justify the extra cost of custom grips, do as advised above- send your gripframe to the custom maker for fitting.
It's the ONLY way you can be sure of a perfect fit.
Denis