Ok, I guess I can see that about summoned equipment although that's not what I think of for +1 every 3 levels because in your way and the way the chart works, you don't hit +2 at level 4 as you do in mine, you hit it at level 6 which is NOT every three levels. And the chart doesn't show +6 for level 18 as it says in the text. Ok, so what about bound items? How does this work?
<emphasis mine>Bound equipment is masterwork quality, and gains a cumulative +1 enhancement bonus for every odd armorist level possessed beyond 1st, increasing to +10 at 20th level. A piece of bound equipment’s enhancement bonus cannot exceed +1 per 3 levels (minimum: +1, maximum: +5); all enhancement bonus beyond this must be traded for special qualities.