I believe it's quite common Karen because you know what you are saying and hear it in your head - but to anyone only hearing it, without knowing already what is being said it is a different thing.
I remember that at a point where I could understand much of what Chris was saying, but most others could not, I was in a store. A lady entered in a wheelchair and a man turned with an exclamation of delight at seeing her. They began to communicate and I could not understand a single thing she was saying. Whatever her medical issue, her speech issues were very different to ALS. I realised quickly that there was a combination of him talking to her about a familiar topic (he was asking about family members and things they were doing), and being familiar with understanding her speech issues.
If I had needed to have a conversation with her I would have been in as deep a pool of water as most people were trying to speak with Chris. I began to realise I could also interpret for him because I understood the answers he might be giving about where things were at with him.