Some versions of the toolkit come with additional utilities, such as a KMS (Key Management Service) host installer. This feature allows users to create a local KMS host for activating multiple Microsoft products across a network.
The most plausible explanation: The number gives it an air of authority and uniqueness, tricking users into downloading it over the vanilla 2.5.x releases. microsoft toolkit 2500
The only option left was a grayed-out button at the bottom of the screen: Some versions of the toolkit come with additional