Normally Informix (and as far as I know all major databaes) are case insensitive in their SQL. Maybe this is an exception.
In Informix you normally can change thsi behavior by setting the DELIMIDENT variable. In that case Informix will assume all qouted strings as case sensitive.
In the past I looked at this, but decided not to use it. Nowaday's I'm sometimes using it when communication with other databases, when they expect IDS to be case sensitive.