Why does TTax allocate 50% of my duplex tax and insurance cost when I itemize along with 100% of the same tax and insurance costs, i.e. 150% of total tax and insurance?

TTax offers option to allow TTax to allocate tax and interest to personal use and biz use but when I enter $5000 in interest, and 50% personal and 50% rental cost, TTax claims I can deduct $7500.  Is that right?