No it doesn't seem to be a grain issue ( that's caught me before! lol) I have had some saywers that I have worked with question some of teh nest I give them. They end up with a sheet left over after changing slighty the nest I give them and also in side by side comps with other nest programs.
It's a small point but as materials and margins get tighter saving a sheet here and there adds up.
I understand completely...yiou gotta save everywhere you can.
Once in a while I get some weird nests. It will nest a single part per sheet for a few sheets then continue on correctly. I can change from vertical to horizontal (or vice versa) and back and it seems to cure it.
In fact, the sample nest I posted had the first two sheets with just one part each. Those parts were tiny and would easily have fit on other sheets.