The distinction is subtle, but in some ways, quite important.
If an action is untraceable, it's impossible to determine where it came from or who did it. This implies a level of anonymity, in that you can't name the person who carried out an action. However, it may be possible to trace an action back to a certain identity without being able to name that identity. Consider a criminal investigation - a serial killer may have a particular MO, or an infamous thief may have a calling card that they leave at the scene. There may be evidence that allows the police to work out how the perpetrator got in, how they committed the crime, and how they got away. In this case, the action is certainly traceable - but the police still don't know who committed the crime, so in that sense, the action is anonymous. In the online world, this is fairly commonplace, with online identities forming a calling card for your actions. If you use some suitably secure method of authentication, you can communicate with someone in a manner that is traceable in that they can be certain that it's you who sent the message, but it's anonymous because they don't know who you actually are.
On the flip side, your online identity may be well-known, and you could use a disposable mail system, or ToR, or hack into a computer and leave a signed message, so that the recipient knows that it came from you, but they can't prove when or where you sent it from - so that message isn't anonymous, because they know it's from you, but it's untraceable because they can't tell when or where the message came from. If you hadn't told them that the message was from you, it would also have been anonymous - but if you chose to claim that the message wasn't from you, nobody would be able to refute that claim.