Since the Deathrattle have the resurrection mechanic, it doesn't seem impossible that the Skaven might have some reinforcement mechanic. 2 of these 5 could simple be "Clan Rat", 1 wound wonders, which can be replaced at the start of each turn. Leaving two "Stormvermin" and the leader as the rats to upgrade.
I saw this image on the WU Facebook page and even with increased speed it's hard not to compare them to the Reavers. They must have some whacky mechanics. Surely?