Are you sure the unit is the same? Maybe Transmissions is not using GiB units?
It happens, that 5.45 GB = 5.08 GiB <--- Note the "i".
The difference between GB (gigabytes) and GiB (gibibytes) is that GB are based on power of 10, and GiB are based on power of 2.
1 GB = 1000 MB
1 GiB = 1024 MiB
1 MB = 1000 kB
1 MiB = 1024 kiB
GiB is not a SI unit, though SI standard recommends to use it for data size. There is a short article http://en.wikipedia.org/wiki/Gibibyte and there is a detailed explanation of this idea: http://en.wikipedia.org/wiki/Binary_prefix
Note that 5.45 GB = 5450000000 B = 5.08 GiB, so if Transmission uses GB instead of GiB, then everything is okay ;)