I guess I don't quite understand regression lines?

I have to do a lot of regression line calculations for work on Excel.

For the past four years, I’ve been using these Sodium Chloride (NaCl) standards that my boss had me make for conductivity readings.

I’ve noticed over time that the readings have drifted. Boss suggested I create new standards. So today I created a whole new set of NaCl standards.

After I made up the standards, I checked their conductivity. They are as follows:
(mM is the concentration – millimol)
1000mM = 57.5 ms
100mM = 8.12 ms
50mM = 4.29 ms
10mM = .941 ms
1mM = .1346 ms
DI H2O = .0093 ms

I created a standard curve out of those numbers (X axis being mM and y axis being ms), then used the regression line equation to calculate the amount of mM I really had – and this is where I got confused.

I got back:
996 mM
128 mM
61 mM
2 mM
-12 mM
-14 mM

So I thought, “Uh oh, something’s wrong.”

Without understanding exactly why, I had a hunch that maybe the 1000mM was too high of a standard. So I removed that and just made the graph from 100mM and down.

It got better although 1mM was half off – it was 0.58 mM.

Then I made a graph just using the 10mM and 1mM values . . . . and it calculated it as EXACTLY 10mM and 1mM.

So. . . huh???? Which graph is telling me the accuracy of my standards?

Also, I looked back to four years ago and found that my conductivity values for each standard was different. As one example, back then I got 74.7 ms for 1000mM.

I used the same exact procedure I did four years ago as directed by my boss. Even if I did do something wrong – like I zoned out while making the standards at some point – my question still remains as to why the calculated values for mM is different whenever I remove an x and y value from the graph. How can I know then the true value of what I’m measuring?

(I also played around with removing the DI H2O values but it doesn’t help)