33 MHz (and 66 MHz) are just round numbers. The real clock limits are 30.0 ns (Table 4-5) and 15.0 ns (Table 7-3). Or, 33.333.. MHz +0/-100% and 66.666.. MHz +0/-50%. Oscillators with normal crystal tolerances are OK ... nobody should care if you are 0.01% over 33.33333 MHz.