esp changes unexpectly

hi:)

  For every ni command  after  this instruction : mov eax,(esp) ,  the
value of esp     changes to be 0x0c from 0xbfffxxx, I can not find the
reason .and I do not know if this is a bug .

 how gdb get and then return the esp value?
 Does it entertaiment register variables ,like eax ,ebx ... by itself
, then copy those values from inferior's register value ？or it
directly return the inferior's register value ？
How to know the internal details for this topic ?

Thanks!