Why is the device added from the console but not added through Virtual Machine Manager?

Question

KVM

virt-manager 1.4.0, qemu 2.7.0-r7, libvirt 2.3.0 From time to

time an incomprehensible garbage occurs - you connect a USB device - a docking station for reading HDD, a USB drive - but it is not visible in virt-manager! Add Hardware->USB Host Device - and it's not there. You fill a small XML file (found in tyrnet):

<hostdev mode='subsystem' type='usb' managed='yes'>
<source>
<address bus='2' device='6'/>
</source>
</hostdev>

(bus and device look at lsusb), then:
virsh attach-device <машина> <файло>
- everything passes. Why is that?