City are without a win in their last four games in all competitions and have failed to keep a clean sheet in any game so far this term.
The England No.1 bemoaned the Premier League champions' luck at times this season but insists they have enough quality to maintain a title challenge.
“Sometimes goals go in against you,” Hart told The Daily Mirror. “That’s football, isn’t it? Things have been falling to people, like the one in the 1-1 draw against Arsenal.
“Joleon headed it away and he [Laurent Koscielny] smashed it in. That’s the way it is. It could have hit the bar and we would have kept a clean sheet.
“So we have to be optimistic. We know we’re doing the right things and we’re tried and tested.
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“I’m being honest when I say that I really don’t know [why City are conceding so many goals]. We’re not doing anything different and the players are very honest.”
The 25-year old also reiterated his disappointment in City's defeat at Real Madrid in their opening Champions League tie of the season and has challenged his side to cut out mistakes if they are to taste European success.
“I feel that way everywhere we go, whether that it’s the first round of the FA Cup or the Bernabeu,” he added. “You play to win. There’s no part of me that’s happy how we lost.
“We played bloody well and there were moments we can all look back upon - Yaya [Toure]’s goal, Alex Kolarov putting in that ball nobody could defend - but we lost.
“The margins for error in the Champions League are much thinner. In the Premier League I wouldn’t say you can necessarily get away with losing, but you have 38 games.
“In the Champions League you only have six and we’ve already lost one. I hope the expectations of us in the Champions League are realistic and that we can fulfil them.
“It’s really hard, though, especially - I imagine - once you get through to the group stages. Then, it’s one mistake and you’re done.
“We’ve got to be ourselves and be what we’ve been. We’ve accomplished a lot but there are a lot of hurdles still ahead.”