The Premier League champions have failed to keep a clean sheet during the current campaign, conceding nine goals in six matches.
"It is inexplicable – I really don't know why things have changed," said Hart. "We are not doing anything differently. Sometimes goals go in. That's football, isn't it?
"We are tried and tested. We closed games out last season, but then people were getting blocks and shots were hitting me. It isn't a case of having no luck, it's just how it is."
City have the chance to keep their first clean sheet of the season on Saturday when they travel to Craven Cottage to face Fulham.