reorg rebuild is going to allocate/deallocate space in the segment as it progresses.
Without more details (eg, table/index sizes - see Bret's post; segment space usage *DURING* the running of reorg), I'm going to guess that reorg was allocating new space for an index, ran out of space in the segment, generated the error, and then rolled back the most recent space allocations for the segment (ie, free'ing up some space in the segment).
This could explain why, when running your query *after* reorg failed, you'll see free space in the segment.
You should be able to see the segment fill up by a) repeatedly running 'exec <dbname>..sp_helpsegment myseg' while reorg is running and/or b) placing thresholds on myseg to show space usage as it's consumed ... to include a threshold firing for the last chance threshold. (If you already have thresholds on myseg ... did the last chance threshold fire and place a message in the ASE errorlog?)
NOTE: I'm assuming you don't have a space accounting issue with this particular segment; that's a whole 'nother discussion.