There is no info on how the curve should continue. Adobe gives you three (or four more on that later) choices in these cases. The choices are miter, which continues the path straight after information stops, round, which makes a circular join, and bevel which just connects the parts at the unknown.
Image 1: The three choices provided by Adobe when information stops, in green.
Now obviously there is a 4 choice. Make the curve yourself. Then you could use whatever you like, there is a infinite number of choices.
Image 2: 3 other choices that era possible.
In practice we can guess a natural continuity for the curve. CAD apps can do many variations*. You can also find a extend curve tool in astute graphics toolset (their site is offline for a moment), unless you want to do so manually They allow for 3 different types of extensions (see here). Its just that these aren't always what is being asked for.
In practice its a good idea to draw the outer shape first and offset inwards as that does not have the problem. You were given 3 choices if you dont like them make your own.
* In practice they have a better ways to get the natural continuity because they use NURBS curves most likely which have more nicely behaved natural modes.