Critical Reasoning (CR) | Re: The prairie vole, a small North American grassland rodent,

Actually E is the only answer which explains a difference in mortality. However, I do not understand one thing here.

If snakes are active from spring through early autumn, probably killing a lot of young voles, how can the group be bigger in winter (after the snakes killed the voles)?
I think that's a flaw in the answer...
The other answers are not better, but this made me stuck...