this is due to a slightly confusing element in the rules. In this case, the rule is probably less than, or 1 less than, and a one is being played. There aren't any cards less than 1 though the rules just let you play anything. The same happens with 10s.
oooooh, I had assumed that "0" is both lower than 1 and greater than 9 and only when a "0" has been played, any other number is greater than and less than at the same time.
PS: Does this mean that if the rule is +1, I can place anything on a "0" card? I was assuming I have to place "1" on "0" on +1 rules. [edit] ah yes, looking at ai's behavior anything goes in that situation...