For me, after each battle I get at least 1 ArchDemon in my reserve slot and sometimes if I have a slot or two one or more additional troops. That's why I have so many ArchDemons in reserve.
It depends on which troops you summon, although, it seems like the description is not accurate with respect to how many you get. I remember having Executioners in my reserve slot and it seemed like I'd only get 1 or 2 even though I should have been getting more. I also seem to remember getting 2 ArchDemons every now and then.
The bottom line is that it should be adding units to your reserve slot, but I think they have to be summoned creatures (not sure why I'd get ArchDemons, though, unless it is counting the Demonologist spell - which I haven't used in a really, really long time).
Hmmm... does Phantom work?
/C\/C\
|