Sensitivity of a frequency meter

The answer to your question, in spite of lack of information about your measuring device, is yes. You cannot measure frequency with an accuracy of 10 mHz unless you can resolve the frequency to at least that amount, preferably even better like in the range of 1-5 mHz. By the way, in engineering we generally use the term "resolution" to indicate how small a quantity a measuring device can display. Sensitivity is usually reserved for how small a signal a measuring device needs to provide its accuracy and resolution. Also note that accuracy and resolution are not the same. Many meters that provide lots of resolution, that is display many digits, do not necessarily have the accuracy commensurate with that resolution.