We prorate the transfer limits to the end of the month if you sign up after the end of the previous month. This is separate from your billing period. Because you signed up toward the end of the month, you will have a prorated transfer limit (27GiB) until the end of the month, when it will reset to 50GiB.
We are aware that there is confusion around this and are working to add some info text in the section if the transfer limit is prorated.
Please let me know if you have any other questions!