This is because there are an awful lot of pages in our system being processed at one time and because we can sometimes discover problems which require extra work before we can make a newspaper available.
We currently only publish complete years of newspapers, so if any pages are missing or not of good enough quality, we will hold back the publication of that newspaper until we can correct it. This is why you might find that 1912 and 1914 editions of a newspaper are added before those from 1913.
We also operate a very flexible scanning process, so we can be nimble if a publication needs to take priority over what we have already planned. This means that the timings for publishing specific newspaper titles can sometimes change at short notice.