Hi. the lvl groupings in arena are 4-6, 7-10, 11-12, 13-14, 15-18 and 19-20. But there is a +- diff in arena lvl, so what happened to u in arena isnt strange. meaning 2 lvl 6 can meet 2 lvl 7 if not enough demands within respective grouping. This is somewhat idiotic but we have to live with it. hahahaha.
Hope u understand what I mean. :)
and P.s What Fizaa says... Dont buy too much gear until u get to lvl 9. most weapons and armour before lvl 9 is just waste of gold. so try and lvl up to 9 as soon as possible. Good luck