I did. ap aging report was set to "as of: March 2019" but the value is always higher than the one in balance sheet

If you click "Aging Options" there is an "Aging As of" option which should be 3/31/19 as well but sometimes defaults to Today's day

Did you check under Dashboard/SetPreferences/Analytics' Report by Period = All Reports?

I followed both of your suggestions but no dice. there's still a difference..