The USB Specification frequency tolerance for full speed devices is 0.25%. We have no way of knowing what is the actual tolerance of devices used to build the boards, and, in fact we have no way of knowing whether the devices were even specified with the correct load capacitance for the ATmega. "Typical" inexpensive resonators might have tolerances of 0.5% or even more, although tighter tolerances may be available.
Regards,
Dave
Footnote:
We also have no way of knowing whether the crystals used on some of the Arduino boards that we buy are specified at the correct load capacitance, but even with rather large errors in load capacitance, frequencies are typically within 100 ppm or so.. Atmel's data sheets tend not to be very specific about crystal load capacitance. (I checked the ATmega8U2 data sheet some time ago and it said the load capacitor values should be somewhere around 12-22 pF. That's the same range given for ATmega328, '1280 and '2560 devices. Digital IC circuitry used for oscillators with external crystals or resonators tend not to be particularly tightly controlled by manufacturers, and values vary from batch to batch. As a matter of fact my observation of several '328 boards that I have built is that the frequency is actually some few tens of parts per million high (or even a little more) when using 22 pF capacitors. Other batches of chips may have slightly different characteristics.