Matching FETs with different feeds of voltage

I've built this circuit and downloaded FET matching spread sheet xls file.
http://www.axtsystems.com/index.php?option=com_content&view=article&id=46:1176lnfets&catid=34:1176ln&Itemid=62

The schematic shows a positive 10vdc. In the spreadsheet file, on the page "match 1", you can set the value of the positive voltage "RD".
The default value is 11,82v or something.
I have measured my FETs with both 12vdc and 10vdc positive rail, and changed the value in the xls file respectivly.
This offcourse  gives different Id values, but the FETs seems to match much better with positive 10vdc than 12vdc.
What gives?

The Id curves should be a simple function of Vgs and not change significantly with Vds. The 10k resistor means it will saturate at 1mA for 10v or 1.2mA for 12V Vds. So I would expect the curves to be a little funny near those upper limits. Perhaps see if your results settle down with a 5k drain load resistor.

In general you should match them for the actual voltage and loading they will be used at in final application.

JR

You are absolutely right. The trimpot drifted. Must get a better one..