Web shield reports a virus, File shield says "No threat found"

Avast Free Antivirus / Premium Security
1

Is it a bug?

When downloading these two files by Firefox…
http://nyerguds.arsaneus-design.com/cnc95upd/cc95p106/cc95v106c_r3.exe
http://nyerguds.arsaneus-design.com/cnc95upd/cc95p106/cc95v106c_r3_u2.exe
… the Web shield warns about Win32:Evo-gen[Susp]

But if I disable shields, download and then scan the files by a context menu, then Avast reports “No threat found”.

2

Win32:Evo-gen [Susp] = Suspicious / is a on access detection only and will not show in any scan

3

But why?
First Avast denies me the file because it is a threat, then Avast says there is no threat.

As a BFU I don’t care about any differencies between on-demand, on-access.
I just see Avast does not behave reliably.