The default data percentage while you create a new RAID is 95%, therefor you will see only 5% space left. Thus, you have maxium "5%" space that you could decide to expand.
In RAID information, you may see 95% for data and 5% unused, that means 95% of total space was devided for your data usage, it does not mean the size amount of all your files. Please mention that the data space could be expand only and could not be reduced. Maybe you would like to know, if 5% was not decide for data purpose, what could it do? It could be devided for iSCSI usage or Target USB (*1).
(*1) Target USB function is limited to N5200 series and N5500 only.
You may also interested at following articles,