Updated FAQ (markdown)

FAQ.md

@@ -1,6 +1,9 @@
 ### Why my 010 replicated volume files have different size?
 
-This is normal. The compaction may not happen at exactly the same time. Or the replicated writes failed in the middle, so there is a deletion file appended following the partially written file.
+The volumes are consistent, but not necessarily the same size or the same number of files. This could be due to these reasons:
+
+* If some files are written only to some but not all of the replicas, the writes are considered failed (A best-effort attempt will try to delete the written files).
+* The compaction may not happen at exactly the same time.
 
 ### Why weed volume server loses connection with master?