/gamerule values can literally be set to anything.

When setting the <value> of a gamerule, using "/gamerule <rule> <value>", <value> can be set to literally anything. To test this, set any gamerule to any value you want, then type "/gamerule <rule>" to check the value of the rule. It should read "<rule> = <value>".
For example, I typed "/gamerule keepInventory toggle", knowing full well this wasn't going to do anything. Except that when I typed "/gamerule keepInventory" it told me that "keepInventory = toggle". I don't think that's supposed to happen.