Fresh out of the box, you're correct, iOS 8 requires 2+GB of space on your device. You can continue below for a more detailed explanation.
There are actually two details that should to be addressed here. First, Apple, like nearly every other company that reports storage capacities, claims 1GB as 1 billion bytes rather than the accurate 1.024 billion bytes. This is maybe a non-issue since it has been a universal trend for years now.
To answer your more specific question, every device and computer comes with some advertised capacity HDD or flash storage. Some of that is used by the operating system. This means there is less actual formatted capacity available to the user.
iOS 8 requires 2+ GB of space, but this can be as much as 5GB if you are updating OTA from a previous version. This is because the OS downloads an installation package before installing itself on the device. Once installed, the installation package is removed. You can avoid requiring the additional space by downloading an iOS update on the computer first and installing using iTunes.
Hope that helps answer your question.
