There will be others providing more qualified answers, but IMHO, although the purpose of the loan is construction, as a single advance, the calculations would not be. I believe the term construction loan is for multiple advances, where the amount and timing of the advances are unknown. Basically the program calculations assumes that half of the principal will be outstanding over the life of the loan, which is why the APRWin is about half of what your calculations are. I believe to qualify for a construction loan there must be multiple advances.
If you used the APRWin, but used the regular rather than construction module it should come a lot closer.
[subject to correction/modification by Richard, Randy, Dan, etc.]
Integrity. With it, nothing else matters. Without it, nothing else matters.