My xperia is not rooted,but i have iroot app!what to do?

A

AC Question

I bought an xperia z from someone else and in app menu i found super user app,but the device wasn't rooted.and root checker app confirmed it.and i cant uninstall that super user app because its system app!and i tried many apps to root again like vroot,kingroot,...
but not worked.so I reset my phone.but now after reseting still i have super user app and still not rooted.i even update that app and it became "iroot'app.but still not rooted.what can i do now?
Sony xperia z c6603,android 5.1.1
and how is this even possible to have superuser app after reseting??!