Federal Tax Liability appears to be wrong by about $2k

I've prepared my taxes in 3 different software packages this year.  The result is that Turbo Tax is consistently wrong.  Two other popular software packages calculate my Federal tax liability at about $6200.  Turbo Tax is telling me that my liability is only $4500.  I cannot seem to figure out where the difference is, but it's clear to me that Turbo Tax is giving me some kind of credit that the other two popular consumer tax preparation software packages are NOT.  What can I do to ensure that Turbo Tax is doing my return accurately?