No factuur in January?

Hello, 

I have only had my NS Flex subscription and OV-chipkaart for 4 months now. I usually get my payment receipt on the 15th of every month. However, in January, I haven’t received an email about the payment so I went to MijnNS to check. It says that my next factuur is going to be sent on the 14th of February and it includes the subscription costs for both January and February. 

The costs are all correct but I’m curious about why they skipped a month? Is it common practice? 

Thanks in advance!