Arduino pH sensor pro kit inaccurate

I purchased the pH sensor pro kitmake_clickable_callback(MAGIC_URL_FULL, ' ', 'https://www.dfrobot.com/index.php?route=product/product&product_id=1110&search=sen0169&description=true#.Vl07WPmqqAx', '', ' class="postlink"') and after doing software calibration I'm getting reading of around 4.03 in a 4.01 solution and 10.05 in a 10.0 solution, both readings within the .1 tolerance, great I thought!. However when I move it to a 7.0 buffer solution I get readings of 7.15 and 7.19 which is outside the .1 tolerance which was disappointing but then I placed the meter into a cup of tap water that I have lowered the pH to 5.9 (confirmed with a bluelabs pen) and I've received readings of 6.55 and 6.66!!! I've left the probe in the glass of water for quite a while now to make sure the reading had time to stabilize but it just keep climbing to the 6.55 value.

What might be wrong with the meter?

I did the software calibration as instructed here:make_clickable_callback(MAGIC_URL_FULL, ' ', 'https://www.dfrobot.com/wiki/index.php/PH_meter(SKU:_SEN0161', '', ' class="postlink"'))#Method_1._Software_Calibration

This really sucks because the pH I am targeting to maintain is 5.8 and if the sensor is this unreliable at that level this is useless to me