This is actually by design, because 'other' is available to you even if you have flagged the post for something else. For instance, you might flag an answer as 'not an answer', see that it was edited 10 minutes later, then flag as 'other' to let the mods know that your previous flag should not be considered valid any longer.
Similarly, you could also flag as one reason, then flag as 'other' to give additional context (e.g. 'you should look at this user's history as well, because he's always writing smelly one line answers').
What we could do is make sure that 'other' is the only flag available in these circumstances, which I believe is a bit more intuitive. I also distinctly remember the system behaving in this manner (if you flag a deleted post, you only see the 'other' reason as one of the options) - this could be a regression due to some recent major overhauls.
I'll status-* something this accordingly once I find out more. Thanks for the report :)
Update
This does appear to be a regression, 'other' should be the only option that you see if you click 'flag' on a post that has a pending flag from you for any reason other than 'other'. Jarrod will have a peek at this and get it fixed.