Why does my filter system seem to be adding contaminants?
It is important to consider that water is often stored in various tanks after being extracted from the aquifer but before reaching your taps. Such tanks are in the municipal system and also inside your home - for example, the roof tinaco. If water from different taps has been stored in different internal tanks, then it is possible that it was extracted from the aquifer at different times and so contamination levels may vary.
This is often the case with "whole house" filtration systems, which often have their own storage tank. These filtration systems do NOT remove arsenic and fluoride (see the next point), and water coming out of such systems can contain different (and sometimes higher) levels of contaminants than normal tap water taken at the same moment. This is probably because the slightly different storage systems of the tap and "filtered" water samples means they were extracted from the aquifer at slightly different times.