My view of this is that the district office had no need to write your passport number or anything else on the back of your KR.2 and KR.3 certificates, but I stand to be corrected if I got this wrong.
The marriage certificates are meant to be valid for as long as the two of you remain married, not only until the expiration date of your current passport.
If, however, you asked for copies of the certificates for the specific purpose of proving to immigration that your marriage is still valid at this point in time, the district office may have chosen to note your passport number on the back as a favour to the immigration office, which this particular immigration office may even have asked for this to be done.