In Section 5.2.7 it says:
"The value attribute is not provided in the bit mask since this is handled in the AccessLevel and UserAccessLevel Attributes."
(BTW: value attribute should be italic and CamelCase)

This statement is only true for Variable, not for VariableType.

Solution discussed in TelCo 15.07.2008:
Add a field to BitMask of WriteMask and UserWriteMask called ValueForVariableType.
This allows specifying if Value is writable on VariableType but does not apply for Variable (thus set to 0 for each Variable!).