Wood – What’s the significance of orbit diameter in a random orbit sander

There is something...in those pieces of wood. (Detailed enough for you?)

Ok, so I would have initially said temperature until you mentioned it was only specific pieces, which leads me to think moisture, but even that would cross pieces...

My best suggestion is sand those spots down, clean them with a solvent dampened rag, lacquer thinner maybe (don't pour it on) and then apply a wood treatment to the spots. Then re-coat.

Edit: To answer your question in the comments

Yes, should have mentioned that, you will need to sand before completing the steps I outlined. If it isn't real wide spread I would just use scraper to get the big chunks off and then a fairly fine grit sandpaper and do it by hand. If there is a lot I would go with a palm sander or belt sander. You don't want anything that spins it will leave marks that will be visible in the final finish.

You'll probably want to rent a proper floor sander. A small random-orbit will take ages and you'll probably burn through most of its useful life.

The sander should come with a variety of paper grits. As with any woodworking project, start with the heaviest and transition to the finest. The final should be somewhere in the 100-120 grit range.

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How far you need to go depends partly on the stain color--is it markedly different from the current color? Is it dark? Dark stain is more likely to compound if it's not completely removed. You'll see the old stain through the new as dark streaks or patches.

If you go the route of renting a sander, the work will be easy enough that you can get through the old stain without much effort, and you'll be confident of achieving a good result.