From: new DBA in '09 on

Question: Why does index fragmentation for one nonclustered table
index vary so much by filegroup partition?

Background: We have several very large tables. One of those tables,
"SalesEntry," has a partitioned index. Each index partition is in a
separate filegroup, and each filegroup is on a separate hard drive.
(Forgive me for including the obvious; partitioned indexes are
somewhat new to me.) Results of sys.dm_db_index_physical_stats shows
me greatly varying fragmentation percentages for some of the index
partitions. Although most of them are similar at 99%, there are a few
with less than 50% fragmentation.

In theory, why would some of the other index partitions have far lower
fragmentation percentages?