- there were gaps in the emails noticed by the parties involved; and
- the system automatically purged itself every thirty days.
I had always assumed all the emails were just lying there on the server, and thus a hacker could pick them all up at once. If they were purged every 30 days, the hacker would have had to come back at least every thirty days, which seems very unlikely. Much more likely is that we are seeing the sporadically gathered collection of an insider, who kept the emails he had access to as he built up evidence of the moral swamp that was the Clinton campaign. It is likely that the decision to leak the emails only came after the cumulative effect of all the corruption and skulduggery turned a true believer into a Clinton skeptic.
The usual criticism of WikiLeaks isn't curation, but dumping everything holus-bolus without taking sufficient care to ensure innocents aren't harmed. 'Context' would hardly make the awful things the Clintonistas commonly did look any better (and note that the missing emails are described as neither particularly interesting or uninteresting). The obvious conclusion is that the missing emails, plus the 30-day purging, points directly to a leak by a non-omniscient insider who collected parts of the email streams over a period of months.