🚀 go-pugleaf

Gnuchessers:

I've noticed the following, and can find no explanation for this:

Set the depth for gnuchess evaluations to, say, 6 ply.
Now set post and go to evaluate a position.
This should yield some score and a best move.

Now in the new position, set the depth to one fewer ply, 5.
Again, press "go" and watch for a score.
Compare the new score with the old one.

They're different!   Why?

This doesn't always happen.  The scores (accounting for
who goes next times minus one) are usually within about 1/10
of a pawn.  But shouldn't they be exactly the same?

By the way, book is off, transposition table is off.

Mike White