Mennonite Church Books - Missing Records

I have noticed that West Prussian Mennonite church books are often missing records - in the sense that records of events that "should" be in there are not.  I have noticed this in particular in the Danzig Flemish Mennonite church because they have records for several centuries (not because the church books were poorly kept but because they have records that span several generations so you can trace families).  For example, Elder Peter Epp #15887 (1725-1789) and Catharina Wiens #15890 (1730-1776) were married in her parents' village of Nassenhuben, which was part of the Danzig parish.  But their marriage on 24 April 1751, is not recorded in the church book, yet marriages are recorded every year for 1667-1808, including for 1751.  But this also appears to be the case in the Tiegenhagen, Ladekopp, and Heubuden church books.

Perhaps I do not understand all the criteria by which events were recorded.  Maybe the elder forgot some events if he only compiled the records periodically.  But it sure is a mystery to me why some events are missing.

UPDATE - Glenn Penner pointed out that both Peter Epp and Catharina Wiens in the example above were baptized in Tiegenhagen, so they were not members of the Danzig church.  They would have been married in Tiegenhagen, which explains why their marriage was not recorded in the Danzig marriage records.  And the Tiegenhagen church books was burned in a fire in 1781, so the records for their marriage did not survive.  But there are many other examples where expected events are not recorded in Mennonite church books.