It took the narrator quite some time to understand that what he heard as 'piper', in fact, meant 'paper' and the phrase 'mind the gap' in the Tube. What do you think caused the problem?

The narrator had the mentioned problems with his language when he shifted to US because in the beginning it was a new language for him to speak. He was not habituated to speaking fluently in any language other than his own mother tongue, i.e. Bengali. Also, the pronunciation was quite different from how it was spoken in London, where he stayed before moving to US.