Moron, You are measuring two different things.
Look at it like a pipe providing your house with water.
Following your example, When they tell you speeds of 25 Megabits per second, They are telling you the CAPACITY of the pipe to transfer data, in other words, how wide the pipe is. It is like 25galon per minute vs 5 galon per minute. The wider the pipe, they faster they can transfer water to your house. On the other hand, when they tell you 5GB per month, they are not talking of CAPACITY but VOLUME. That is how many GB total.
Think of it like filling a lawn pool. The pool can hold up to 5GB of data. And you can fill it up with a hose that can transfer 25MB per second. You can use a larger or a smaller hose, but once your pool is filled up with the 5GB of data, it is filled up, you have to wait for next month (another empty pool).
The telco is trying to profit two ways: by selling you a larger hose and by selling you a larger pool. It used to be that they would only sell you a larger hose and you could fill up any pool-size you wanted to. Now they are limiting your pool size too.
I know, it sucks.