You can 'tweak' crystals to a very high tolerance.
I used to design pagers, and we bought 2.5 ppm crystals from NDK by the million. Not good enough for the job.
The circuit included a series L ( a few hundred nH) and variable C ( a few pF ) tuned to around the wanted (3rd overtone) frequency, and the crystal could then be easily pulled to <0.01 ppm on test.
We used to tune about 0.05 ppm high, as the crystal would settle down that much over a year or so.
Of course we needed a reference much better than that. The industry standard is 10MHz, and I think we had a caesium source at that to lock all our signal generators. 10^-10 or so?
Allan.