The traffic sent through your device might fluctuate due to multiple reasons. One of which is the simple laws of demand and supply, which are affected by the rapid inflow of new users (more supply) or less demand from our customers.
We are constantly balancing this effect on our end, but it depends on the magnitude of the supply/demand change.
As an example, just before Christmas users might be looking for cheapest prices from e-commerce sites and - users will see higher traffic usage. After new years - present have already been given and demand fades out - traffic decreases.
You can read more about this on our blog post here.