Not all threats in the bowtie will have the same level of significance to the operation.
Differentiating threat significance according to exposure serves the purpose of highlighting where the areas of greatest concern lie.
Not all threats will necessarily have the same significance to all stakeholders.
Some threats may only be encountered on a limited basis (e.g. take-off with flying surfaces contaminated by ice), while others will be a potential issue for every flight (e.g. taking off with the aircraft loaded outside of the correct Centre of Gravity).
The exposure rating is considered before these particular controls have their effect.