This makes sense because with just your income you were in a lower tax bracket and when you entered your husband's income, (W-2) you went into a higher tax bracket.  The higher the income the higher the taxes and when you have higher taxes it decreases the refund that your program initially calculated with just your income.

In addition, you may want to ask your husband about the number of exemptions that he is claiming on his W-4 at work. The more exemptions on a W-4 means that less taxes will be withheld from paychecks throughout the year, but come tax time, (April 15) the tax bill will be higher than it would if there were 0 exemptions on the W-4.