Sanders campaign started April 30. In this Politico story, the campaign stated they had raised $13.7 million from April 30th to June 30.
"76 percent of the Vermont senators donations were $200 or less, amounting to just under $10.5 million of the $13.7 his campaign raised through the end of June.
The $15.2 million comes from a transfer of $1.5 million from his Senate campaign committee.
There were several stories and campaign announcements regarding the amount that matches the report:
Campaign Finance Institute has two charts that support the reported numbers:
At the end of the day, nothing "mysterious" but a staffer making a mistake in the paperwork.