A Petrobras honlapján ezt találtam, valószínűleg ez az oka:
Temporary Diesel Price Subvention Program06/01/2018
Rio de Janeiro, June 1, 2018 – Petróleo Brasileiro S.A –
Petrobras informs, in addition to the material fact disclosed on May 28, 2018, that its executive board, at an extraordinary meeting held yesterday, approved the company’s accession to the first phase of the economic subvention program for the commercialization of diesel, established by Provisional Measure No. 838 and regulated by Decree No. 9.392, both dated May 30, 2018, and extended to all producers and importers.
Thus, besides the voluntarily reduction made by Petrobras, on May 24, 2018, the company's average selling price at refineries and terminals, in cash and without taxes, will be reduced by R$ 0.07, to R$ 2.0316 per liter as of today. This reduction will be maintained until June 7, 2018, the end of the first phase of the program, with no change in the period originally set for the voluntary price reduction made by Petrobras.
This difference in price, between June 1 and June 7, will be reimbursed in kind by the Federal Government, through the National Petroleum Agency (ANP), within the scope of the program, in accordance with the regulation in force.
The accession to the first phase of the program does not bind the company to the second phase, which the decision will be submitted to the evaluation of the competent authorities, after publication of the regulation.
In addition to the reductions of R$ 0.23 and R$ 0.07 practiced by Petrobras, the average sale value will also be affected by the reduction in the tax burden of contributions to PIS, COFINS and CIDE-Combustíveis, from R$ 0.16 per liter, governed by Decree No. 9.391 of May 30, 2018, since the taxation of diesel is concentrated on the producer or importer.
Facts deemed relevant on the subject will be timely disclosed to the market.
This report on Form 6-K shall be deemed to be incorporated by reference into the Offer to Purchase dated May 21, 2018, relating to the previously announced tender offers by Petrobras Global Finance B.V., a wholly-owned subsidiary of Petróleo Brasileiro S.A. – Petrobras.