The computer cheats!

[EatYerGreens]

I lift the seige and I am currently under threat of excomm - ther is no justice!!

When your forces are inside the castle, note that the province around it has changed colour to the invading faction. It's now their land, so your siege-lift, in fact, counts as an attack on them!

Pope says "get out within 2 years" but, by winning the province back, you've achieved that bit - it's not their land any more. Now all you have to do is abide by the "no further attacks for 10 years" bit and all will be okay.

The purpose of the warning is to give you the choice of politely pulling out and handing the lands back, or pressing on with ending the siege inside 2 years - e.g. one year of attrition and assault in year 2 - but make sure you win that one, or else!

What really gets peoples' goat is where some other faction starts the war but it's you which gets given the warning, as per what happend to KHV.