Car Insurance

I've been with Progressive for years now. I started with them, because they were the only ones I could do all of my account management online. Now, just about all insurance companies do this. Since I pay in monthly installments, I get hit with added charges of about $80 extra every 6 months, which brings my total cost to $739.99/6 months. I decided to start looking around for other options. State Farm worked out to be pretty equal to Progressive's price. Geico would save me about $100 every 6 months, so I started to get optimistic about their service. Then I decided to get an online quote from esurance. Their online quote can't be right. It worked out to be $338 per 6 months. How on earth can they provide me the same service as these other companies for $400/6 months or $800/year less? Something doesn't add up, but I think I like esurance's math a whole lot better than the others.