That actually shouldn't be happening. But if extensions are added and then the projects are played around with (extensions are deleted again or removed via WinZip / WinRAR), then it can happen that certain blocks can no longer be assigned (because the corresponding extension is missing). The same can happen when importing a project from AI2Offline to AI2 and vice versa.
See e.g. here:
This block 
does not exist in AI2 but in AI2Offline.
And this block 
does not exist in AI2Offline (but in AI2).