This happens a lot and I have tested it A LOT. I have come down to the AD networks are giving away a freebie or two....to get you to push the offer even harder.
Everyone else's points are factors too...but it makes sense for the AD networks to do this in order to get more people pushing more traffic.
I've thought bout this, but didn't think they would really do it. Other people think they do this shit too?
If they do, it's working, because I usually test small and if it's profitable, I increase bids and spend quite a bit. More often than not, I get a shit load more traffic, but go from 4-5 to 0 conversions a day.