Welcome to the group.
That error message occurred on the recipient's end -- don't worry
about it because we all received your original question (you should
have received a copy of your own message as well so you know it went
through).
To answer your question, I found this page
http://homepages.ihug.com.au/~parsog/photo/sensors1.html
It appears the reason for this notation is historical. Some of those
mixed unit/decimal fractions are irrational in every sense of the word
(mathematical and otherwise).
