Waste water returned after test less then sample size

Hello

Some context :

I clean the probe/calibrate the KH director every 3 months with Hanna special cleaning for PH and with GHL PH calibration. I also test with calibration solution and I noticed my results are way off ( even with 100 ml and multiple tests to purge any last drop of salt water). I had to use an 18% correction factor. I have the KH Director for about 18 months.

I am wondering if the problem is not the probe only . I noticed that when I get sample size to 80 ml at the end of the test I have 73 ml water + regent(measured with special liquid measuring cilinder) . Also related to size/volume of tubes etc I followed the info in the manual and when the remaining water in the tubes if flushed back at the end only 2-3 cm of tube still hold water.

Why do I have 73 ml wastewater after using 80 ml sample ?