# Why the reconstructed jet need to substract the “background”?

There is a trouble bother me a long time. I know it may be stupid question. But I really need your help. It's in the CMS paper:

CMS Collaboration, CMS Physics Analysis Summary No. CMS-PAS-HIN-12-013, 2012, visit http://cds.cern.ch/record/1472734

In the section of jet reconstruction, it already contain the jet selection process when reconstruct the jet. More specifically, in this case, the minimal transverse momentum of all charged particles was set to 1 GeV/c. And the transverse momentum of a jet is required to be larger than 100 GeV/c. There are also restriction on $|\eta|$.

So, after the selection process, the constitutions of the reconstructed jet are the particles that we need.

Why we still need to substruct the background from original reconstructed jet? After the selection, why still exist the underlying event or what are the constitutions of the background?