Why does Ubuntu 13.10 show 1GB more free space than Android 4.4.2?

Linux:
http://farm4.staticflickr.com/3768/1...a1a715be_b.jpg
Android:
http://farm3.staticflickr.com/2879/1...b02699e3_b.jpg

Looking at the new 16GB/12.5GB Nexus 5 which T-Mobile sent me
to replace the almost useless 4GB/600MB LG Optimus F3:
http://farm3.staticflickr.com/2825/1...849b4ff3_h.jpg

I'm now done installing necessary apps into the internal memory:
http://farm8.staticflickr.com/7437/1...b9c07b48_b.jpg

It seems, after the Android OS was installed, there was roughly
12.55GB of the original 16GB, so the OS took up about 3.5GB.

The pre-installed apps took up another few hundred MB such
that the 16GB Android 4.4.2 phone started with about 12.28GB:
http://farm3.staticflickr.com/2918/1...bd07a0ed_b.jpg

Then, after I installed a dozen or so basic smartphone apps:
http://farm8.staticflickr.com/7437/1...b9c07b48_b.jpg

The phone was left with about 10.72GB of available space:
http://farm3.staticflickr.com/2879/1...b02699e3_b.jpg

Note: There is absolutely no user "data" yet; we're just
talking about pre-installed and post-installed basic apps.

So, just "my" list of basic apps alone, seem to take up (roughly)
about 2GB of storage space (this includes offline road maps for
California, which are a necessity, IMHO, for when you really
need it when there is no cell signal or your data plan is low).

However, when I plug in that 12.5GB Nexus 5 into Linux,
Ubuntu Saucy Salamander shows "free space" of about 11.5GB,
which is actually 1GB too high as compared to what Android
4.4.2 shows.

I'm not sure why.
Do you have experience with such things?