Notice our trader windows don't look like yours? Well, ed and I posted our own trades using 4 to try to replicate your findings and then took a pic of our 'your offers' window...
Looks like ed went further and also tested out using 2, 3, & 5 and his next post reflects that when he went up to 6 it began to calculate the number of stars correctly, but only if all items are 6 and above.