UNION ALL results - forum topic by lgscheurich

OK.  I'm stumped on this one.  I run a query in Teradata, and I get the following results:
 
Measure      Group  Count
Measure 1   XXX     33450
 
That's all good.  However, when I include that query as part of a UNION ALL query that, that particular row now looks like this
 
Measure      Group  Count
Measure 1   XXX     33580
Measure 2   YYY      25810
 
I know there is no overlap in the data.
 
What would cause the number to change like that by simply adding a UNION ALL and another query to this?
 
Any suggestions would be much appreciated