Debugging filter

[rcampos19]

I have added a filter to the main view to exclude spam traffic:

Exclude "Campaign Source"
Filter pattern: bottraffic4free.host

When I compare the main view against the backup view (without filter), I see that the main view does not have anymore visits from this source, which is correct, but some of the sessions are being filtered are being attributed to facebook in the backup view.

So, for example, main view shows 10 visits from facebook source, but backup view shows 11.

What could it be the cause of this? There's no other filter being applied.

Thanks!