Why Cube Partition "Auto Slice" Does Not Work As Desired Usually?

"During processing of MOLAP partitions, Analysis Services internally looks at the Source Query and identifies the range of data that is contained in each partition by using the Min and Max DataIDs of each attribute to calculate the range of data that is contained in the partition. The data range for each attribute is then combined to create the slice definition for the partition.
The Min and Max DataIDs can specify a either a single member or a range of members. For example, partitioning by year results in the same Min and Max DataID slice for the year attribute, and queries to a specific moment in time only result in partition queries to that year’s partition.
It is important to remember that the partition slice is maintained as a range of DataIDs that you have no explicit control over. DataIDs are assigned during dimension processing as new members are encountered. Because Analysis Services just looks at the minimum and maximum value of the DataID, you can end up reading partitions that don’t contain relevant data.
For example: if you have a partition, P2003_4, that contains both 2003 and 2004 data, you are not guaranteed that the minimum and maximum DataID in the slide contain values next to each other (even though the years are adjacent). In our example, let us say the DataID for 2003 is 42 and the DataID for 2004 is 45. Because you cannot control which DataID gets assigned to which members, you could be in a situation where the DataID for 2005 is 44. When a user requests data for 2005, Analysis Services looks at the slice for P2003_4, sees that it contains data in the interval 42 to 45 and therefore concludes that this partition has to be scanned to make sure it does not contain the values for DataID 44 (because 44 is between 42 and 45).
Because of this behavior, auto slice typically works best if the data contained in the partition maps to a single attribute value. When that is the case, the maximum and minimum DataID contained in the slice will be equal and the slice will work efficiently.
Note that the auto slice is not defined and indexes are not built for partitions with fewer rows than IndexBuildThreshold (which has a default value of 4096)."

Reference:  SQL Server 2008 R2 Analysis Services Performance Guide