Updates faster than reads

I am doing benchmarking for mongodb (university homework) and perform some operation sets. The problem is updates are faster than reads for example:

2000 ops:

just updates: 0m31.247s

just reads: 0m48.996s

How is this possible? I encounter no errors and updates are performed correctly. I use a db with 3 shards.

There is not enough information here to answer your question.

What are the updates?  What are the reads?  Are both using indexes?  Are both operating on the same number of documents?