An executive at Steward Health Care System told presidents of Steward's hospitals Monday that the Boston-based chain "unilaterally withdrew" from talks to acquire Mercy Health System in Maine after concluding Mercy had misrepresented its finances. The account given in a confidential memo from Steward director of media relations Chris Murphy, obtained by the Globe, differed from a statement Steward and Mercy released jointly late Friday afternoon saying the letter of intent to merge had been terminated because the parties "were unable to come to a definitive agreement" during negotiations.