I am on a 5GB/month wireless broadband plan from Optus. For the purposes of selling the plan to you, Optus define 1GB as 1000MB (see screen shot below).

For the purposes of billing you, however, Optus define 1MB as 1024KB (also see the screen shot below). This means that they would define 1GB as 1024 MB for data used. Hence the 5GB plan is actually a (5000/(5 x 1024)) x 5 = 4.88 GB plan.

5GB should give you 5 x 1024MB = 5120MB.

Given that excess usage (not that we are at any risk of that) is charged per MB, not shaped, it would be good if they were consistent.