Making Sense of Cellular Usage Data

I thought I knew how the tracking of cellular usage data was recorded by the iPhone, but apparently I am mistaken.

Last Saturday I took a road trip and on the side, I decided to see how much cellular data would be used while I had my iPhone in map navigation mode with Siri giving directions. It was approximately 3 hours of driving roundtrip. From my home location, I recorded the "Sent" and "Received" values in the "Cellular Usage" section of my iPhone settings at the very beginning of my road trip; then again when I returned to the exact same home location.

I expected that either or both of the Sent and Received values would have increased noticeably when I returned home from my road trip. My impression being that the cellular usage data is simply a cumulative total of cellular bandwidth used since the last reset of the data (my last reset being July 31).

However, the results I ended up with are as follows:

Sent (initial,final) = 31.2 MB, 29.7 MB
Received (initial,final) = 352 MB, 341 MB

As you see, for some reason the values "decreased" and not evening by very much. I am mostly very confused, however, by why the values decreased.

I also just took a look at what the values are on my phone right now, which are as follows:

Sent = 33.0 MB
Received = 352 MB

Suffice it to say, I'm at a loss in understanding the cellular usage data reported on the iPhone. I did this experiment to start with so I could precisely track and measure the amount of cellular data usage for specific Internet usage functions; e.g. map navigation, emails, texting, etc.

Feedback and insight on this is much appreciated.