Tablespace Usage Query

We are getting different result while we are trying to find the tablespace usage with below query for one tablespace.

Query 1:-

select df.tablespace_name "Tablespace",

totalusedspace "Used GB",

(df.totalspace - tu.totalusedspace) "Free GB",

df.totalspace "Total GB",

(100 * ( (df.totalspace - tu.totalusedspace)/ df.totalspace))

"Pct. Free"

from

(select tablespace_name,

(sum(bytes) / (1024*1024*1024)) TotalSpace

from dba_data_files

group by tablespace_name) df,

(select (sum(bytes)/(1024*1024*1024)) totalusedspace, tablespace_name

from dba_segments

group by tablespace_name) tu

where df.tablespace_name = tu.tablespace_name and df.tablespace_name in('XYZ');

Query 2:-

select b.tablespace_name, tbs_size SizeGB, a.free_space FreeGB , tbs_size - a.free_space UsedGB