This is realated to the following discussion in the forum

https://groups.google.com/forum/#!topic/mongodb-dev/IMhn83pLbtI

 
The bytes currently in cache is more than bytes read into the cache. This sometimes is even more than the collection or index size.
 
"cache" : {
                                "bytes currently in the cache" : 111408914,
                                "bytes read into cache" : 73171386,
To reproducethe issue i have included a simple program and the data together with the mongo config here.

https://drive.google.com/open?id=1dAQNaFIkG9LwNXD23C-OPX6ojqAYg72a

As the results suggest the bytes currently in cache is more than the total index size. around 20%.