Could it be due to the effect of the "derating table"?
Check if ODT setting is correctly in two cases?
Thank you for the replies.
Actually i think the problem was due to an error in the ibis file: hyperlynx required that for clks, address/ctrl, strobe and dqs to have different "model Selector". At the moment i think that was the problem... but i need yet to double check.
ODT configurations are OK in both configurations.