Why OS X says a low free space, despite having 25GB free?
25GB is not alot of space on a nearly 500GB drive. It seems that OSX has a built-in alert for 5%.  

Recall that the operating system needs to rely on the drive being available to manage conditions where there may not be enough memory to complete processes. This is what we call 'virtual memory'. OSX will swap older memory items to the hard drive if needed. 

In addition, OSX wants enough space to provide the ability to write all of current memory to the hard drive in case of a power issue, so that you do not lose data due to battery issues. In this case, it needs at least enough space to ensure it can write 4GB/8GB/16GB depending on your RAM size.  

All of these items are managed by the OS, and if it detects a condition where it may not be left with enough space on the hard drive to perform its functions, it will alert you the user.