Bogomips != frequency. 388 equals about 400MHz with this CPU and operating system. The N800 will change the frequency at will, depending on the load (and sometimes it'll drop down the voltage too, all to save on battery). It also has to go down to a lower frequency when it's using certain DSP (digital signal processor) operations, because the DSP must run at a frequency which is linked to the CPU frequency. If the CPU frequency is 400MHz then the DSP frequency must be reduced instead of increased, or it would go above its design limit. Nokia had this table in a presentation posted here and elsewhere:
As you can see, sometimes the ARM CPU frequency must be reduced in order to let the DSP run at its highest performance.