IGPU systems on Windows usually set aside a fixed pool of memory for the graphics usually set by the platform. As in, if you need more video memory you need to go to the BIOS and reconfigure it. When you look at total system memory available in Windows it's the total installed minus however much you defined for the video memory. If you need to have the GPU mess with something you still have to a copy it from the system side to the GPU side. Applications can run out of video memory while still having available system memory.

It's only been somewhat recent that I've personally seen much hardware that allows for that reservation to be dynamically defined, and not with any Intel integrated graphics so far.