1 / 8 * 3600 * 24 * 30 = 324000 so that 1GBit/s server could conceivably get 324TB of traffic per month "for free". It obviously won't, but even a tenth of data is more than the data included with the 10G link.
The 10GBit uplink is something you need to explicitly request, and presumably it is more limited because if you go through the trouble of requesting it, you likely intend to saturate it fairly consistently, and that server's traffic usage is much more likely to be an outlier.
[1]: https://lowendtalk.com/discussion/180504/hetzner-traffic-use...