If I recall correctly, this is envato’s explanation of the stated sentence:
Subscriptions get auto-renewed every month, and that is what is meant with “started”. So it is not only people who first registered in April, but also whose subscription was autorenewed or still running in April.
The reason for this is the way earnings are distributed along authors on elements. Always a period of one month is looked at, but it will start at the date of registration of the buyer, not necessarily the first of each month. So if someone has registered on the 30th of a month, basically everything they download in between April 30th and May 30th will be shown in the sales report for April. This is also the reason why the sales reports take that long to generate.
Now I’m actually surprised that there are any earnings for April (which basically means any one-month period between April 1st and May 31st) given that Video has landed on Elements at June 5th. So my guess is: there has been some testing with only a few buyers going on in that period that generated those earnings. Hence the low numbers. May report will be better, but only June report will be the first one to consider relevant as it is the first one that includes all active subcription periods falling in a time where video was actually available to everyone on elements.