Summary Plan Descriptions Under Scrutiny discusses how, despite the disclaimer language, courts will not always enforce the plan document provisions if they are different than the SPD provisions:

Most Summary Plan Descriptions ("SPDs") contain disclaimer language stating that in case of conflict between the SPD and the Plan documents, the Plan documents will always govern. In fact, sometimes SPDs are given less attention than we benefits lawyers recommend, usually due to cost or other factors and because people often believe that the disclaimer language will protect them. However, whatever the reason, it appears that the courts are not always willing to enforce provisions of a Plan document that are not disclosed in the SPD, despite the disclaimer language.

We also discussed this issue in SPD Language Providing Less Plan Benefits Overrules Plan Document Language.