When storing numeric fields, using an odd number of digits allows the sign to be included in the storage area without wasting any space. 

For example the number -1234567, when packed as 7 digits is stored as follows. 

----
1357
246- 
----
thus taking up only 4 data slots. but when packed as 8 characters it uses 5 data slots 

----
02467 
0135- 
----
or when 7 digits, non-packed, it will be stored as 8 data slots, including the sign. 

----
FFFFFFF6 
12345670 
----
