Maybe I don't get it well, but, does "half-size" is for to have an A5 booklet printed on A4 home printer ? If so, when I submit, i should have page divided by two, where here it's added some extra page instead.
That should be right -- on an A4 sheet of paper half size would print two zine pages per side to make an A5 booklet. The extra pages could be if your total page count is not a multiple of 4. If not, ZineArranger adds some blank pages in to bring it up to a multiple of 4, since there are 4 pages total per sheet of paper.