Interesting. I had a couple of bills last week for an aircraft we owned in 2011 that was removed from the German register at the start of 2012.
Nine months after that I had abill for €130 for Frequency Allocation in 2011, which I queried. Eventually I found a gentleman who explained what it was for, told me that the LBA have no obligation to issue any documents in anything other than German and also that they were "entitled" to issue bills for 2011 until the end of 2015.
He didn't say that they could/would issue multiple bills for the same year! I paid that one but I will be querying these two subsequents.
My German is also not good enough to decode the technicalese on these bills....