How is it that I end up owing taxes at the end of the year when I claimed 0 exemptions on my W4?

I only worked 20 hours per week at my job, but my husband has a much higher paying job (also claimed 0). When we crunched the numbers from my W2 with his, we ended up owing a couple of thousand dollars which we wouldn't have with just his income alone. I don't understand. Please help!