Many people report not remembering therapy sessions or details well, sometimes much larger chunks. If you want to test it, perhaps try to use one of those programs that can transcribe audio. If the words are mumbled and hard to hear, the program may not detect it either but if you see it written, that could be more convincing.

Do you ever experience dissociation? That could be, although it would be a bit unusual for it to be so specific to just single words. I sometimes zone out in meetings when the conversation does not grab my interest enough or simply when there is too much going on in my head, and can miss parts of what was said and then bring up the same thing that was already mentioned. It has annoyed some people, so I try to make a more conscious effort not to let myself zone out, but it is not always that easy. This is not dissociation, just distraction, and my own thoughts can often be the most distracting. I don't tend to have any other issues with attention or memory.

If those words are real and they somehow qualify you or your experiences, and if you tend to worry about how others perceive you, it would not be too surprising to block them out. A lot of memory processes are not conscious at all.