You forget about the second proposed rule:

• Only if by pay-out time the curator bucket holds a negative RSHARE count, this negative count is filled up to zero with positive RSHARES from the author bucket .

The idea is that at pay out time a "negative" curator bucket is filled up to zero with what remains in the author bucket first. So at 1000 and -900

• the author bucket receives 667 - 300 = 367 rshares.
• the curators bucket receive 333 - 600 = -267 rshares. This is negative and needs to be filled up to zero if possible at pay out time.
• After filling up the negative bucket to zero, there is 100 rshares left in the author bucket.