While these are using the same settings the way greyscale and the other modes work is fundamentally different, hence the different results. Greyscale varies the power according to the darkness of the image, and generally gives unpredictable and undesirable results, because as the laser scans along it is (by nature of using the greyscale mode) burning areas that it’s just burned, which get darker and hotter, and this means it’s more likely to bring the sap out of woods as you have found. This issue is compounded when resizing an object as this changes the max speeds reached.
The other modes dither the design into off/on commands (all at full power), and generally produce much better results. I recommend finding your image settings using stucki or jarvis.It will give you much more predictable results.
Here’s a bit more info about image settings more broadly:
It’s a very big topic though, and one of those wicked problems where each setting affects the results of multiple others. Images engravings can be really frustrating, essentially.