I think some people are actually voting for their favourite here, not considering which one is in most need of an expansion.
You should rename this poll to "which of these games are your favorite".
The results are not indicative of what Paradox "should" focus on next at all.
I have a feeling that people are voting for EU3, simply because it's an option. By reading the comments on here, it seems that EU3 has the numbers, but Rome has the desire and enthusiasm.
This is not a poll asking what game we think needs an expansion to meet its potential. This is a poll asking us what game we would most like to see an expansion for. Rome needs the expansion the most, and I don't think you'll meet much resistance on that point. But many would like to see expansion packs for other games besides Rome for various reasons: They don't play Rome much, they like the era of other games more, etc...