The OP said his mother sent a letter of explanation, so it is likely that the mother WAS provided with an opportunity to respond to the embassy's concerns regarding what was construed as a misrepresentation. They would not summarily reject her on that ground before asking her to explain what may have caused her to provide the information she did.
Unfortunately, "the consultant/person helping her made a mistake" explanation is regarded by CIC as right up there with "the dog ate my homework". Every applicant is responsible for the information he/she provides to CIC, and it is not enough to blame someone else if they didn't do it properly when they were assisting you. That is why you sign a declaration stating that the information provided is accurate to the best of your knowledge and you understand that you will be liable for the consequences if it is not.
I agree with you in general.
The reason I asked my questions is that being in a process of completing those applications for my parents (who do not speak english) I am being really paranoid - I would not want to cause any misrepresentation accusations but in some cases my parents have to recall some information without any supporting documentation (e.g. some really old addresses or months of address change), so they try to recall the best they can (e.g. recall whether it was in winter or any associations with past events). Another example would be membership in some youth organizations or trade unions (which in old times were almost mandatory in our home country during certain periods of person's lives - I am talking about Eastern Europe, soviet times). Nothing to be concerned about or hide about these 'formal' memberships but we can only try to 'guesstimate' months years based on typical ages etc.
ALso, my parents are not sure about birth places for their parents (who are all deceased) - and they do not have documents either...
So, what should we do to avoid misrepresentation accusations - write the explanation for every month/year or unknown information that they are not sure about (that would be painful)? OR just say something in general that for this and this piece of info my parents tried to do the best to recall the information or that we do not know/remember? We have no intention of lying - just do not always know and have no supporting documentation.
That is why I was kind of curious what the orginal poster's issue was and what they deemed material enough to be misrepresentation (but I understand that it is personal and the poster may not reveal it)