Transcription errors!
 

Just spent ages looking on 1921 census on FindMyPast for my great great aunt Kate. Her husband had the unusual name of Gerrance so I looked for him first. Tracked them down by using the name of the road.

He is down as Terrance (despite also being recorded as the householder Mr G Chapman, the house is down as Trevilder instead of Treviledor (image is quite clear) and their daughter Katie is down as Kathie although again the image is very clear.

I know handwriting can be difficult to decipher but in this case it's all quite clear.

I worked through the 1921 census looking for about 1100 people and submitted over 300 transcription error reports. There are still around 100 people I did not find who must be there somewhere but so mistranscribed I could not find them what ever I tried. So my experience there is at least a 25% error rate which is so bad that they are not coming up on a search.
Like you Nell, many of them were quite clear on the image.