As is sadly common amongst practically every hardware manufacturer - the quoted memory size is not the size that is actually available to the end user. But Microsoft has taken this to a whole new level.
A 32GB surface actually works out a 29GB and then the OS+pre-installed apps take up 8GB and recovery takes up another 5GB. So you only end up with 16GB free not the 32GB you may have been expecting.
Similarly the 64GB version, only has 46GB available.