That bug has been opened on https://launchpad.net/distros/ubuntu/+source/gcalctool/+bug/67408

"The real value of pi = 3.141592653589793238...
gcalctool thinks pi = 3.141592653
The value has been rounded down, it should have been rounded up.

The value in the source is longer than displayed in the GUI:
./gcalctool/get.c: init_constant(3, "3.1415926535"); /* pi */
So init_constant() seems to be rounding incorrectly.

The value in the source has also been rounded incorrectly, but that may not be the problem.
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http://librarian.launchpad.net/4921664/gcalctool-5.8.24-ps1.diff
Correction to rounding of pi.

Hi there..

I have attached a patch to fix the second issue. (The value of PI as mentioned was confirmed in Wikipedia, which may or may not be definitive :-)
...
While I can confirm the new Pi value is rounded more correctly, I am not sure it makes any difference. I think we need to figure out what type the Pi value is stored in, and see if 3.1415926536 is already too many significant digits for that type."