Why was Dr. Chanda puzzled? What was unusual about Bepin Babu’s loss of memory?

Bepin Babu decided after talking to Dinesh Mukerji and Chunilal that he had a very specific partial loss of memory. Therefore, he told his doctor that he forgot only about ever being in Ranchi. This mysterious memory loss got Dr. Chanda thoroughly confused.

Usual memory losses do not happen to such a particular memory as in the whole memory of ever having been to Ranchi. With all the other memories- short term and long term intact, it's highly unusual that Bepin Babu would loose only the memory of Ranchi.