In the same way that good attacks will go on barren runs, good defences will leak goals. It's all about the relative strength of each.
For example Real Madrid, have a worse defence than us. 4 more goals conceded in 1 less game. Yet they are at the top. Why? Because having Ronaldo, Bale, Benzema, Isco, James Rodriguez and Morata gets you shitloads of goals (almost double our total), that's why.
If you have that level of quality up front, you don't need the best defence in the world. It's all relative, it's not rocket science.