"MEMORY_BUSY" - when is it thrown or printed?

What surprises me: I do not seem to consume that much memory (measured with process.memory():

before I start drawing:

 { "free": 10203, "usage": 1797, "total": 12000, "history": 70,
  "gc": 0, "gctime": 23.59008789062, "blocksize": 15, "stackEndAddress": 537087280, "flash_start": 0,
  "flash_binary_end": 582576, "flash_code_start": 1610612736, "flash_length": 1048576 }

after drawing:

{ "free": 10197, "usage": 1803, "total": 12000, "history": 70,
  "gc": 0, "gctime": 23.52905273437, "blocksize": 15, "stackEndAddress": 537087280, "flash_start": 0,
  "flash_binary_end": 582576, "flash_code_start": 1610612736, "flash_length": 1048576 }

Thus, there seems to be (and should be) plenty of memory left?

Or does the graphics buffer consume a lot?