This is perfectly normal behaviour, yes.
An N900 with standard factory settings (no underclocked or overclocked kernels or anything like that) will switch between a couple of frequencies, depending on how hard it has to work. So if your N900 just sits in your pocket doing nothing (but checking mail every once and a while for instance), it will run at 250 MHz, while when you start up a lot of applications or one heavy application, it will switch to a higher CPU frequency to handle that application.
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