> spigot can never determine the next correctly rounded digit after "0.1234" with certainty, for reasons elucidated in the documentation.
Because spigot rounds towards zero in that mode, and it can only bound the result as arbitrarily close to 0.12345 - i.e., it can never decide between 0.12344999..99something and 0.12345000..00something because the "something" part that might break the tie never appears. This is a general issue with exact real computation; in more complicated cases, it can even be undecidable whether some expression is exactly zero.
Because spigot rounds towards zero in that mode, and it can only bound the result as arbitrarily close to 0.12345 - i.e., it can never decide between 0.12344999..99something and 0.12345000..00something because the "something" part that might break the tie never appears. This is a general issue with exact real computation; in more complicated cases, it can even be undecidable whether some expression is exactly zero.