It could be one of three things. First, as an economic class applicant, you are lumped with FSW applicants, most of whom DO have to show proof of funds - it could simply be a generic letter.
OR, it could be as SenoritaBella suggests - additional proof of employment.
Worst case, they suspect something is not on the level. For example, someone on here talked about having an "employer" report wages to the CRA (he paid the taxes out of his own pocket), even tho he was not actually working or being paid at that time period. That's fraud, and it happens.
In any case, you need to bring what they ask for.