That was not supposed to happen. Just recharged the Men's Shed pocket WiFi 4G internet modem for the first time. Such devices, on the chosen network, come with 4 GB of data with an expiry of 14 days, which became due tomorrow, so I applied a $40/60 day prepaid voucher that's good for 4 GB.
The opening 4 GB/14 day allowance is not supposed to carry over any unused data.
But it just has done so. The remaining 2.55 GB has been added to the new 4 GB/60 day allowance, so it's now a 6.55 GB/60 day arrangement.
Colour me pleased.