That is because you can set this timeout also globally in QMC->System->Setup->QlikView Servers->Your QlikView server->Performance->Sessions.
And now the catch: "the lowest value takes precedence", meaning that if your global setting (default = 1800 seconds) is lower than your document setting, that global setting will be applied. I guess this is because of a security consideration.
You can read the QMC help on that setup page for more information.
BTW Leave the Document Timeout value alone. It has nothing to do with Sessions per se, but determines when the QVS should throw this document out of memory.