Compensating for microphone roll-off comes with tradeoffs. The body of the iPod increases the directivity of the microphone response, especially in the upper octave band. Amplifying certain frequency bands also amplifies the noise floor within those bands. Additionally, the inexpensive electret microphones in iOS devices cannot be expected to exhibit very tight tolerances in their frequency response curves. The response of a particular microphone element may also be altered by leaving the iPod in a hot car.
The goal of using a built-in mic on an iOS device should be to get a reading in the ballpark. It seems to me that you already confirmed that to be the case in your experience.
If accuracy is most important to you, I recommend that you use a proper measurement microphone, which can be connected to the iPod with suitable adapters.