I looked through several catalogs and only found 20 ppm crystals (or worse).
I'd bet those clocks do suffer drift, though small. Probably have some sort of crystal oven. Aging is another issue - as crystals get older, they drift.
Note that accuracy and drift are two different things. A 20 ppm crystal is spec'd to oscillate + or - 20ppm when using specific load caps at a specific temperature. The crystal frequency will change based on a number of factors - temp, age, load capacitance,... I believe it is possible to characterize these effects and compensate for them. Proper low noise design of the circuit is important as noise can be seen as a oscillation and thus increasing the frequency.